I emailed my sister recently to ask what she recalled about the year we moved to Paris, a messy, chaotic time just after our mother died and my second son was born. I lived through it hastily, carelessly, my attention dissipated in a fog of grief and exhaustion. Now, nearly 10 years later, I find myself wondering about that lost year; I want to go back and unpick it, remember, reclaim it.

Her reply read: "Wasn't that when Theo was really scared of that big ball in Thomas? That was terrible and hilarious."

Boulder!

She was right. I hadn't thought of Boulder for years, but for a time it was a looming figure in our family mythology. My then two-year-old son was, like so many small boys, an unswerving devotee of Thomas the Tank Engine – the television show, toys and stories – but the episode featuring Boulder was a step too far. Watching it again now, I can see why. It is the story of how Rusty (an obscure diesel engine in Rev W Awdry's canon) is sent to work in a mine overlooked by a precariously balanced, gigantic brooding boulder with a face. Now how could that possibly go wrong?

The inevitable happens, of course: everyone ignores Rusty's misgivings until Boulder is shaken loose by the mine excavations and proceeds to pursue him and various other engines around the disaster-prone island of Sodor at high speed until it is finally halted in a highly cinematic climax, flattening an engine shed, missing Percy by a buffer's breadth, and causing a significant conflagration in the process. The special effects budget must have been off the chart. Even the resolution is lacking in simple feel-good factor: the Fat Controller puts Boulder back on a nearby hillside "but Rusty is sure that on a clear night it is gazing up at the mountain, and that its sighs are being carried on the wind to where it once used to stand, proud and silent". Brrr.

When we first watched this epic of pre-school peril, Theo started to whimper as Boulder wobbled then fell from its perch, but his clenched hand around mine refused to let me turn the DVD off. His sobs turned to howls as the boulder disappeared from sight, then appeared suddenly, thunderously glaring, behind the beleaguered Rusty, rumbling towards him at high speed. As we reached the final burning shed sequence (by halfway through, I had reasoned that it would be better if Theo watched to the end to see for himself that it didn't end in catastrophe; a calculation I acutely regretted), I could see myself paying for years of intensive therapy, for both of us.

Thomas the Tank Engine – particularly the televised version – is full of catastrophe. Chains break, loads drop, there are crashes and fires and derailments, all pictured with that eerie, stop-motion clarity. It's a harsh environment, too, the Island of Sodor. Strictly in the interests of research, I have just watched an episode where a whole train of coaches falls dramatically into a ravine on a foggy night. No attempt is made to rescue them, and they aren't even mentioned again. It's hardly the Teletubbies.

Thomas the Tank Engine. Photograph: HITEntertainment/PA

Thomas, I think, is one of those children's programmes that many liberal-minded adults submit to rather than enjoy. I find it genuinely unpleasant at times: full of violence and jealousy and revenge, lots of punishment and precious little forgiveness. In 2009, the Canadian academic Shauna Wilton published a critique of the socially conservative agenda in the show: the repression of dissent and change, few and inferior female characters, rigidly enforced class distinctions.

Theo's father, who is French and thus wasn't exposed to the Rev Awdry in childhood, discovered the series in growing disbelief. "Isn't it sort of racist? With the trucks as inferior beings it's OK to mistreat?"

Yet it must tap into some dark part of the small child psyche, for it is wildly, enduringly popular: sold in more than 185 countries, a multi-million-pound cash cow for licensees, the literary estate and the maker of the television series, Hit Entertainment alike.

Some of the things that spook me about the show – the long-held, exaggerated facial expressions of the engines, the early signposting of change (danger), the great clarity of the action and the lack of clutter and movement in the background – are precisely the things that research by the National Autistic Society in 2001 discovered made it appealing, and a key learning and communication aid for a significant number of autistic children.

For my part, I owe Thomas – and yes, even Boulder – a somewhat reluctant debt of gratitude. That's because somehow, they became an entry point into, and an oblique language to describe, the things I was least able to talk about with my son at that terrible time 10 years ago.

For months after that fateful first – indeed only – viewing, our lives were dominated by rock-based peril. The Thomas obsession was not new, but now Theo's interest took on a darker hue. His first thought on waking was to explore how likely it was that we might be crushed by gigantic rolling geological phenomena today. And throughout the day, the sight of anything faintly stony in the streets of Paris would set him off again. It was hard not to find it horribly funny.

"Boulder!"

"I think that's more of a pebble, Theo."

"NO LIKE …" he paused.

"Boulder!"

"No, that one's a scrunched-up paper bag."

"Boulder come squash you." This line was delivered with the terrible weight of prophecy.

In the Batignolles gardens, where we went often to feed the ducks, he would sit rapt on a low wall overlooking the curving sweep of the 10 or 12 sets of train tracks that ran under the Cardinet bridge, watching the perpetual ballet of trains going into and out of Saint-Lazare station and chat about things going bang, naughty engines, causing as the Fat Controller always said: "confusion and delay". In the evenings, as I put him to bed, he would take hold of my wrist urgently and check a last time.

"Boulder coming?"

"No. Boulder isn't coming. It's too big to get in the lift and it can't do stairs."

Theo's obsession with vehicular peril was most apparent as I walked him in his pushchair to nursery each morning, a 20-minute walk across the wide, grey avenues of the north-west of Paris. He would recline against the canvas, wave a plump, rather patrician hand in my direction and instruct me to tell him a Thomas story, usually giving me some opening premise to work with. "Percy fall in a hole," he might suggest.

My stories were, I liked to think, a touch more fanciful than Rev Awdry's usual: my trains were waylaid by dragons, trampled by herds of buffalo or carried away on rockets. Even so, every day, the pattern was the same: I would try to tie up each tale with a neat rescue and resolution, a party with tea and cake, and Theo would refuse, insisting on further, and more perilous, adventures. "But no," he would say at any hint of a happy ending, eyes flashing with a two-year-old's determination, "'cause the lion ATE the wheels."

"Ah. OK. Right, so the lion ate the wheels, but they gave him a poorly tummy and so he had to sick the wheels back up again. And Percy took the wheels, and gave them a rinse, then put them back on and raced off just in time for the …"

An imperiously raised hand would cut me off. "But THEN Percy 'sploded."

The harder I tried to rescue our heroes, the more rococo and alarming the disasters Theo insisted befall them. The spectre of Boulder was never far from his thoughts. Sometimes I would try to fight back. "No, I don't think Percy did explode. It was … a firework in his boiler?"

Theo would not brook this kind of weaselly corner-cutting. "No. Percy 'sploded."

A happy resolution to one of these stories was a hard-won thing, rare and grudgingly conceded.

But in real life bad things could happen: they just had. Although Theo did not know it, my mother had been killed at a station – though not by a train – when a mechanical walkway collapsed with her on it. I hadn't told him that. Actually, I hadn't told him anything.

Theo had been there, though. He had travelled with us on the night we found out my mother had been killed; he had been grasped like a life raft and cuddled and spoiled gratefully by myriad relations. He had come to the funeral and sat on my father-in-law's knee, miraculously silent throughout. Even so, I had not told him, in so many words, that she was dead: I just couldn't. I did a lot of things after her death that I had never imagined I could do, but this had defeated me: I never managed to get my son's butterfly attention for two minutes to say to him, clearly and unambiguously, "Granny died."

I knew I should have. The bossy ladies at the Paris nursery had told me as much, intimating clearly that I would be responsible for incalculable damage to his psyche. Even without their input, I knew I was wrong. You have to be straight with your children and protect them from the heartbreakingly odd scenarios they invent when they don't understand the upheaval and distress around them. "But he never asked," I told myself, as if that were sufficient excuse. Really, I knew then as I know now that the only person I was protecting was myself. Partly, I feared how little her death might actually mean to him and how swiftly she would be forgotten. At 18 months, how could it be otherwise?

But I think that where I failed, Thomas helped. On our trips to and from nursery, with their rambling narratives of rolling-stock catastrophe, we were exploring the uncertainty of his world, peeping into an abyss full of broken trucks and then stepping back from the brink. Things went badly, sometimes terribly, wrong but it didn't mean that everything was broken or hopeless. Sheds were rebuilt, buffers replaced, engines repainted. Theo's challenge to me to find ways back to safety, ways of resolving or subverting disaster again and again began to feel like a convoluted way to make good my failure.

The stories continued for most of the year we were in Paris. By the time we left, Thomas was being edged out of Theo's affections by an equally predictable passion for excavating equipment. The mind-numbing strains of Here Comes a Digger replaced the doom-laden incidental music of the Island of Sodor, and the spectre of Boulder receded, very gradually. We still didn't talk about my mother much – he still didn't ask – but I lost my terror of the subject. Or perhaps the rawness just receded a little. When it did come up, I found I was able to answer simply, without embellishment or artifice.

"She had an accident."

Because an accident was something any Thomas enthusiast would understand.