Four o'clock on a Monday evening, and a straggle of small children are trooping into the Carers Cottage, a restored council building on the edge of Howard Park, Kilmarnock. They've come for their weekly activity session, and for a couple of hours they'll play, do arts and crafts, and be given a hot meal, complete with tatties and vegetables ­ something most will not be getting at home. Some are absolutely tiny. Their hands disappear into their coat sleeves, and when they sit down on vinyl-covered benches, their feet barely scrape the floor. Despite the cheery yellow-and-blue decor and the comforting smell of cooking, this place feels starkly municipal. As if they might need reminding, a brightly painted mural above them shrieks: "Young carers just wanna have fun!" They look anxious and unsure of themselves until Jason, the centre's family support worker, wedges himself between two brothers, Sam and Luke. "So what yous all been doing over half term?" he asks. They shuffle about and say nothing. "That bad? No!" But it's a hard question because the answer for many is not much. All of these children have caring responsibilities at home way beyond their years. Their parents are ill or disabled or they've got a sibling with special needs who requires so much care that it impacts on the whole family. Sam and Luke's mum suffers from depression and their dad has multiple health problems, including chronic arthritis. He recently had a stroke, which means the boys and their two older brothers and sister take on a lot of responsibility. The centre managed to send them away for a week's break in a holiday cottage, donated by a local supporter. It was only four miles away, but there was a huge kitchen and chickens to feed, and it was a change for everybody from the chaotic conditions at home. "So, boys, what was the best bit?" Jason asks. Sam, 8, thinks about it. "There were beds with real lights you could turn on and off," he says, looking bemused.

Tonight, PC Fergie Grant has popped in to talk about graffiti and Asbos.

"Och no, they're never too young to start talking about crime," he says. But later concedes that perhaps they are. This group are all 8 to 10, and some of them are living in situations that make them very vulnerable. But when they stare at him with their round, baby faces, I don't think he has the heart to give them his drugs spiel. "What would you do if you saw someone doing something wrong?" he asks instead. "That's right! Tell your mummy." As he talks, Erin, 8, dressed in pink, with the sweetest face, traces a pattern with a finger on her skirt. Her brother, Finn, 10, stares at the floor.

After being cared for at home for months, their mummy died of breast cancer two weeks ago. PC Grant isn't to know, but I know, and my heart breaks for them.

Jane Smith, the centre manager of The Princess Royal Trust East Ayrshire Carers' Centre, began the young carers' group nine years ago with Laura Bennie. Then there were six children. Now four full-time and 10 part-time workers care for 400 children, aged from 5 to 21, at two centres, one here in Kilmarnock and one in Cumnock, 15 miles away, covering an area of 490 square miles. There are activities evenings every night for each age group, a total of 10 groups. Every time I see Jane, Jason, Lorna, who runs Kilmarnock, or Derek, who looks after Cumnock, they're carrying food. All the meals are cooked on the spot ­ soup, mince and potatoes ­ and they always overbuy, so the children who they know won't have much at home can take what's left away with them. Food is a big priority here, for everything it signifies: warmth, nurture, care. After Lorna got married in August, Jane laid on a mock wedding for all the children. Formal invitations were sent out, and Lorna, wearing an Oxfam dress, "married" wee William ­ a 10-year-old who looks after his disabled father ­ in the back garden with tables laid out, so everyone felt part of it. Jane cooked steak pie and mash and made sticky-toffee pudding and a cake. One of her absolute rules is that carers deserve the best. It's her way of caring for them. The children are never out of her mind. It's a standing joke that Jane cannot pass M&S without going in and buying pants. The office has enough pants, socks, T-shirts and jeans to clothe a primary school. And if she sees toothpaste reduced in Sainsbury's, she'll buy a basketful for distribution on respite breaks when children turn up with just the clothes they stand up in.

At Christmas there are parties for each age group, a huge dinner, and all the children and their brothers and sisters get a sack of presents. Parents who are ill often buy something from a catalogue ­ a pair of boots or a toy ­ but end up handing it over beforehand, so a child has nothing to open on the day. Many won't be getting Christmas dinner at home either; there'll be nothing to differentiate it from any other day. Jane tells a story about an eight-year-old girl, Kerry, who, when her mum was ill, regularly heaved her baby sisters up to the Foregate shopping centre in Kilmarnock in a double buggy. Kerry came to the young carers' Christmas party wearing a pair of thin leggings and a Mickey Mouse T-shirt she was very concerned about. It turned out it was her only Christmas present, and when she got home she had to wrap it up again for opening on Christmas Day. One of the few remaining pieces of furniture in the house was a highchair. Half the floorboards had been ripped up and burnt because there was no money for heating. Someone once criticised Jane for giving some children more than others. "Yes I do," she retorts. "I don't apologise for it either."

Children don't think of themselves as young carers. To them, what they're doing is ordinary. Nearly all of them say: "I want to help." And they do, but it takes an adult on the outside to notice when "helping" has become a workload that stops them behaving like children. Often, the little ones tell you an edited version of the truth. This is partly because they don't really know the facts ­ they're just used to Mummy always lying on the sofa ­ and partly because they're instinctively trying to normalise what they do. But what they do is never normal. It isn't right for a child to care for a parent, and it makes them frustrated, worried and depressed. Eventually they stop going to school or seeing friends because they don't have time to keep up with them. Some of the older ones give their parents their medication, they shop, they cook ­ or at least heat up food for their siblings ­ and they try to keep the house going. Others have a relatively light caring role, fetching and carrying, making tea, but nonetheless, the emotional impact of living with a parent who is depressed or chronically ill is absolutely crushing.

Children who are in crisis tend to do all they can to hide what's going on from the outside world. "My mummy needs to lie down a lot," Isla, 8, tells me at the Tuesday Cumnock group. "She has asthma, so I get her inhaler for her and I try to keep everything tidy." Isla is tiny, with an elfin face.

It's her first time here and she wears a big hat she never takes off. Derek knows the background of every child here. When I tell him something about the mother of one of them, he's baffled and upset at having missed something. He says Isla is an only child; she comes by taxi from the estate where she lives with her mum, who is a drug user. I've seen enough already to know the conditions she probably lives in and the sense of isolation she must feel when the front door closes. Derek spoons out "stovies", a nourishing soup of carrots, potatoes, onion and sausage, and the children all sit around tables laughing and chatting. Next to Isla is Katie, whose mummy died of heart failure at 31. She now lives with her poppa (grandpa), who is disabled. There is nothing here to differentiate them from children in any other social situation, Brownies, Cubs, Beavers. But meet them in their homes and you see children trying and failing to hold things together.

In the corner of the room, a boy with dark hair concentrates on his Hallowe'en lantern while, outside, his estranged father lurches past clutching a can of Special Brew. This boy's mum is also an alcoholic. The centre has supported him, encouraged him and cared about him, and that will continue until he stops needing them. It's been one of the few constants in his life. Children like this are in danger of being lost to society before they've even begun. Without support, they're likely to inherit destructive patterns of behaviour in the way previous generations inherited debt. Out of 400 registered young carers, 125 have parents with drug and alcohol problems, another 100 with mental-health issues. Jane refuses to differentiate. "Children are children. They didn't ask to be put in this situation. If I can help even one to see they can make different choices in their lives, then what we're doing is worthwhile." But what they do is so much more than that.

A report by Stirling University, funded by Lloyds TSB, concluded that the centre was improving children's self-esteem to the point it was actually "breaking into the cycle of deprivation".