Some people's lives are almost unimaginably hard, stricken by bad luck. The welfare state is there to mitigate misfortune, or it was. Emma and Chris Ford have three severely autistic children below the age of eight. Emma was once a well-paid PA, but that life is a world away, as both parents have given up work to care for the children full time. They stand to lose £2,716 a year in the welfare reform bill now being debated in the House of Lords.

The Fords are remarkably resilient, but after her third child was born, Emma suffered a long bout of postnatal depression. The children get harder to look after as they get older and, she says now, "If I'd understood what the diagnosis of the others meant, I wouldn't have had a third."

But she's not complaining – or at least, she hasn't until now. Their two-bedroom, housing association home in Horsham, West Sussex, is cramped for three hyperactive children. Rhys, six, is in special school, a child with no sense of danger, on impulse throwing himself down stairs, pulling furniture down on top of himself or hurtling into the road regardless of traffic. Outside he needs a wheelchair. Barely speaking, he eats with his hands, smearing food everywhere and he needs Ritalin to manage at school. He wakes at 4am every morning and has to be watched every waking minute from then on. As we talk, he sits for hours in a big woolly hat, peering through his thick specs at a repetitive computer game that keeps him calm and happy.

Martyn, seven, manages in mainstream school with a teaching assistant to help: he bounces about the sitting room with the youngest, Caitlin. She has just had a heart operation, wears a hearing aid and, aged three, hardly talks, so she starts in special school soon. Life is a struggle, but a kind volunteer comes in for two hours a week so Chris and Emma can go for walk alone for a brief break. "I can manage. Life with the children is fine for us and I'm not complaining," Emma says. "We do have most things we need and we're not asking for anything more. Just please don't take anything away."

When universal credit comes in, two of their children will lose out as disability additions to their child tax credits are abolished, with only a small increase coming for the third child. Other cuts in the bill, as yet undefined, may take more from them.

Losing more than £53 a week means cutting out the children's extra activities and trips out: special riding for Rhys, dancing for Caitlin, karate for Martyn, who is frequently bullied. Different aspects of the welfare reform bill often mean the same family is hit over and over. The Fords' housing association have offered a four-bedroom house, giving the children rooms of their own, so they wouldn't wake each other up any more. But Emma doesn't know yet if the £181-a-week rent will be covered by new housing benefit rules only entitling them to a three-bed house.

All eyes will be on the Lords on Tuesday night, none more anxiously than those millions on disability benefits watching as the bill cuts its way towards them. Will the Lords rebel again, as they have four times so far? Crossbenchers hold the key, but will they stay up long enough? Many peers are incensed by Lord Freud's clumsy attempt last week to overturn their rebellion late at night after most had gone home.

Today they debate the abolition of the disability living allowance (DLA), to be replaced with the less generous personal independence payment (PIP). The government's tougher new criteria include figures showing that 2.2 million who would have drawn DLA will be cut to 1.7 million. There is no research justification for this – but Iain Duncan Smith, the work and pensions secretary, has pledged to cut DLA costs by 20% – and its overall fraud rate is just 0.5%. This is part of the overwhelming £18bn cut that transforms the welfare state from one that pays according to need into one that hands out according to criteria designed to limit funds, despite need.

Contentious

To shrink the number of claimants, everyone on DLA will have a new medical assessment, which attracts today's most contentious amendments. Nearly two million working-age disabled people will be processed rapidly through what threatens to be a crude check, not by doctors, at the phenomenal administrative cost of £675m.

Many fear the new tests will be as bad as the tests for the employment and support allowance, carried out by occupational health company Atos, where 40% of decisions have been wrong, and overturned on appeal. Some people have died within days of being passed "fit for work".

Baroness Grey-Thompson, a cross-bencher, will put an amendment to ensure that before the tests are rolled out in a hurry, parliament receives an independent report on a pilot sample, with a trial period. She reflects the anxiety of all the disability charities: how fair can a new points system be with a pre-set intention to cut 20% regardless?

The Fords will be among the many watching and hoping the Lords soften the impact of the bill, as other cuts rain down on them already from social services and the NHS. "I can manage the children, it's the admin that gets me down," says Emma. "It's struggling to get appointments for things they need."

Their list of appointments includes a paediatrician, therapists, a cardiologist, an opthalmologist and an audiologist, sometimes two or three a week in different clinics. "They took away the speech and language therapist, that was the first to go – and yet that was the most valuable of all to us. The therapist taught Rhys to speak, and Caitlin needs help. We needed to be taught how to communicate with them, not to ask questions or offer complicated choices they couldn't cope with."

Emma's life is spent on the telephone, arguing the case for her children. It took over three years to get a statement for Rhys to secure his special school place. The DLA form is a notorious 40 pages long, a ferocious barrier for many claimants who, by definition, have difficulties: so far she has had to fill those forms out 12 times for her children. "How would anyone manage who couldn't read or write very well, or doesn't understand the system?" she wonders. "Those people who go for PIP tests will have no idea what to say, or what to do if their money is taken away."

Francis Maude, the Cabinet Office minister, is the Fords' MP, and Emma rolls her eyes when she tells of his doorstep visit at the last election: "I said I was really worried his party would cut benefits for disabled people. He said to me, face to face, 'You know about David Cameron's son, that's why you know there's no way we're going to harm disabled children like yours'. Right there, on our doorstep."

Once disabled children reach the cliff edge of 18, things get worse for their families. Michelle Harrison, in Nottingham, has a son, Pete, who is 19. He too is severely autistic, and can't speak, wash or dress himself. Often he refuses to eat, or get in a car. He goes to college for a basic life skills course, but lost his education maintenance allowance, a precious £30 a week, when it was abolished last term. With acute diabetes, he has to have his blood tested every four hours, day and night, sleeping in his mother's room for fear of night-time seizures.

Pete is on the highest rate of DLA, but Michelle fears he will be downgraded in the PIP tests. It happened once before when a teacher mentioned in a report that he could get himself round school, forgetting to add that it was only with help from a full-time assistant. As a result, he was downgraded to mid-level DLA: "It happens so easily, so randomly," Michelle says.

Staunch campaigner

She works as a carer, a cleaner, takes in lodgers, teaches English as a foreign language and makes hanging baskets. "Anything to keep off benefits, but I couldn't cut back a penny more than we do already," she says. She struggled when the washing machine broke down last week. As a staunch campaigner with disability charity Contact a Family, she says, "I've been an advocate for disabled families for years now, but if I didn't know the system, I wouldn't cope."

One of the amendments would ensure PIP testers ask for medical reports from claimants' own doctors, so they have full records, unlike the Atos work capability tests. "The new PIP tests terrify everyone, because we know what they are for: they are designed to throw a lot of people off benefits," says Michelle.

If she reaches the stage where she can't cope with her 6ft son, it would cost the state £3,000 a week to keep him in residential care. Michelle says she voted for Cameron: "I really believed that someone with a disabled son would never harm us. How wrong I was."

Donna Glynn lives alone in east London, a 38-year-old wheelchair user who was born three months premature and suffers from cerebral palsy. She wants to work, but jobs are scarcer than ever for disabled people, though she has nine good GCSEs and has worked as an administrator. When the bill passes, she is typical of hundreds of thousands who will lose £20 a week when the PIP comes in, as she is not on the top level of DLA.

As a long-term jobseeker, she has just been put on a work programme with private company Ingeus. Unfortunately when she went for her appointment there were steps and no wheelchair access. When someone offered to help, she was told she couldn't legally come in as there was no disabled toilet. But she still hopes they'll find her work.

"Losing £20 might seem nothing in Westminster, but on £115, that makes a lot of difference to me," says Donna. "What can I cut back? It'll be the black cabs that take a wheelchair, which cost £7 one way to see my mum. I'm very stuck and socially isolated; the internet's my salvation, but I won't get out to see people much. Do they have any idea what losing £20 will do?"

Like all the 3.2 million people, old and young, on DLA, she knows the Lords can't overturn the bill – but they can still mitigate some of the damage it threatens.