There are few parenting shames greater than being told the food you are giving your children is not up to scratch.

My third child started preschool while I was living in Tokyo — the only non-Japanese kid in the class — and I learnt the hard way that making a school lunch in Japan is not only about a full stomach, or even nutrition. It is an artform.

One afternoon when I went to pick up my little son, the teacher called me aside. All the staff at the school had noted a problem with the food I had packed that day, he said.

"Sandwiches," the teacher told me, "are not appropriate because they are not healthy."

He took out his phone and up flashed a picture of my son's school lunch. Together we peered at the image: two wholemeal vegemite sandwiches (the jar had been carefully, lovingly, carried to Tokyo from Sydney), a banana, a cheese stick, slices of capsicum and carrot and a home-made muffin.

I had to admit that it looked very yellow.

Japanese mothers make 'character bento' lunchboxes for their preschool children. ( Flickr: luckysundae )

I told him this was a pretty typical lunch for a young child where I came from — so what should I do differently?

The teacher promptly produced more photographs on his phone. "These," he told me by way of explanation, "are some of the lunchboxes that other children brought with them today."

I scrolled past elaborate combinations of food featuring balls of rice that had been crafted to look like Japanese cartoon characters — Hello Kitty, Rilakkuma, Doraemon, Pikachu.

These rice sculptures with sweet faces cut from seaweed and cheese were nestled into an elaborate landscape of food. There were eggs in special shapes, sausages sliced to look like an octopus, "flowers" made from carrot or ham and toothpicks with tiny ladybugs on the end that held together artfully rolled pieces of omelette. Cherry tomatoes and florets of broccoli filled out the gaps.

I thought the teacher was joking: it could not be possible that any parent would produce such a thing for a pre-schooler's lunch. I waited for him to burst out laughing — gotcha!

But the teacher's face was stern.

I had just been introduced to "chara-ben", the character lunch box, and to be honest the learning curve looked steep.

Step One: tool up

A mini industry has sprung up in Japan to support mums making chara-ben — one of those hybrid words the Japanese create that means "character bento"'.

Stores everywhere sell moulds to shape rice into cartoon characters and special hole punches to create faces out of sheets of dried seaweed. There are cutters to accurately slice the octopus sausages and plastic dividers that look like swaying grass, to separate different food groups and keep this tasty box of art looking beautiful.

Japanese mothers create cute lunch boxes for their children using a range of tools. These punches create faces out of dried seaweed.

As well as tooling up, conscientious mums enrol in private classes to learn recipes and design tricks, or subscribe to Instagram accounts and YouTube channels.

Standards are now so high, and the pressure to produce a good-looking lunchbox — known as obento in Japanese — is so intense, that preschool kids compare their lunches and compete over whose is best.

Some are even bullied. I began to suspect that my son might soon be one of them.

Mothers (yep, it is almost always the women) report "obento stress" from the effort to keep up.

Some rise pre-dawn to begin the long process of cooking and creating their children's preschool snacks.

Now on the verge of my own obento stress, I found a course and hurried to sign up.

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Step Two: enlist the experts

One weekday morning I joined a small group of Japanese and foreign women to learn the secrets of kyara-ben.

I discovered that many Japanese mothers believe the beauty of the lunchbox reflects the love they have for their child. It is important for bringing joy to his or her day and encouraging picky kids to eat healthy food.

In other words, my sad lunchbox effort did not just expose my sloppy cooking skills, it implied I was not a good enough mother. Ouch.

Each lunch box is created with a set of important principles in mind: at least four colours should be present (low marks for my yellow-toned effort, then), a careful ratio of protein, starch and vegetables must be planned, and a design created that is visually appealing and yet firmly packed to prevent it from being damaged in transit.

The perfect box, we were taught, comprises about 50 per cent rice, 25 per cent protein and 25 per cent vegetables.

Japanese mothers often make intricate lunchboxes for their preschool children. ( Flickr: luckysundae )

Home cooks resort to a traditional series of lunchbox recipes that include tiny meatballs, slices of grilled fish, crumbed pork and chicken skewers.

The recipes must be strongly seasoned in a variety of textures to make them tasty to eat when cold.

An expert might be able to create one of these kyara-ben in 30 or 40 minutes (a darn sight longer than it took me to slap together the vegemite sandwich). But a typical Japanese mum could take an hour to create a good-looking kyara-ben each morning.

But there's a dark side to the gourmet school lunch

As I sat at my obento class that day another, darker, side to this act of mother love began to take hold.

Despite Japan's reputation for hyper-modernity it remains unsettlingly old-fashioned unsettlingly old-fashioned when it comes to the role of women. Change is coming, but the pace is glacial.

Pregnant women can display these keychains while travelling on public transport to inform fellow commuters that they are pregnant. ( Reuters: Yuya Shino )

It is overwhelmingly women who produce these intricate lunches and that is largely because Japanese women are still pressured to give up work when they marry or fall pregnant. "Mata-hara" or maternity harassment (another example of Japan's quirky hybrid vocab) is widespread and relatively few women maintain careers after they give birth.

Japan's education system builds in an expectation that mothers will be at home and available for regular volunteering at school.

And in line with Japan's tradition for order and cultural uniformity, there is a rigid set of expectations about the way children are prepared for school each day: right down to their names being hand-embroidered onto spare clothing and hand towels for washing up after they have eaten their gourmet lunch.

This culture of stay-at-home motherhood is driven from the top. Japan has very few female MPs and company CEOs are overwhelmingly male. Even Japan's royal leader must be male and any royal women who marry commoners must leave the royal family.

Crunch time is coming

As Japan's population ages and shrinks, fewer workers also means less tax revenue — not to mention more job vacancies.

When combined with Japan's reticence to embrace immigration, only now beginning to be addressed, the cultural restrictions on the participation of women in politics and the workforce makes less and less sense.

Prime Minister Shinzo Abe tried to tackle the problem with his "Womenomics" strategy — designed to encourage women into politics and employment — but six years on, little has changed.

A feminist revolution at the school gate?

I confess that I was rather proud of my new bento box skills, and my little boy got a thrill when I brought it home.

While I lived in Japan I did bust out my chara-ben skills from time-to-time and no doubt the novelty of it did the trick in enticing my son to eat a new range of veggies.

But as I packed my first chara-ben in class that day, I wondered if a feminist revolution at the preschool gate was where change in Japan could be most effective.

And most days I decided that the traditional Aussie vegemite sandwich was not just a perfectly acceptable preschool lunch — it felt a bit like an edible protest against the patriarchy.