I’M A lunchbox spy. I can’t help it. When I sit down for dinner with my children I ask what their classmates ate that day at school in their packed lunches. It’s great family entertainment. We laugh. We cry. And it’s cheaper than a West End show. Try it sometime if you have kids. Borrow some if you don’t.

Steel yourself, however. In my experience the answer to “What did everyone eat at school?” won’t be “nothing”, but it could be “two Mars bars and a packet of crisps”. That’s the lunchtime diet of a six-year-old boy in my younger son’s class. Really.

I didn’t believe it myself until I saw it on a school trip. I used to think that my son was having me on. He knows how to wind up his mother. Then I went to the British Museum with 60 children from our West London state primary, and under the same roof as the Rosetta Stone, that triumph of human intelligence, a Year 2 child in full view of his class teacher ploughed his way through two caramel-filled chocolate bars and fistfuls of Walkers. I almost choked on my multi-grain ham sandwich.

It’s here that I’ll confess to being what passes in the UK as a bit of a health nut. At home we bake wholemeal bread, and I’m one of a dying breed of parents who make time to eat at a table most evenings with my children. Try not to hate me. We have a national problem. Jamie Oliver is a treasure, to be sure. British school meals are a disgrace. Our Government has betrayed us by letting standards sink. But let’s be honest. Too many parents these days are lazy about feeding their children. Send me hate mail if you must, but I won’t back down. I’ve seen too many nightmare lunchboxes.

The dirty little secret of Britain’s lunchboxes, of course, is that food in our country is a class issue. Most of the children eating unhealthily at my son’s school are poorer than those eating well. On a visit last month to a junior school on a housing estate outside Birmingham, only one girl in a group of Year 5s (aged 10 to 11) had a fresh piece of fruit. One of her friends had a Kit Kat, a packet of Hula Hoops and a Snickers bar. Her drink was orange soda. No one had brown bread. Several of the girls were fat. “My mum never buys brown bread,” said one child. “I only like white.”

My friend works for an educational publisher that has access to market research on how families eat; this shows that parents are giving their children low-quality food to eat because they eat it themselves. It’s not ignorance about what is healthy, nor is it always a money issue. Many cannot be bothered to make the effort. Shockingly, many low-income families buy ready-made meals that cost more per person than roasting a chicken. It’s faster and easier to buy ten croissants from Somerfield than it is to split open a pitta bread and fill it with grated carrots and cheese.

Yet it’s taboo to complain publicly about the food in children’s boxes. The social divide at our primary school is acute. I’m seen as one of the “rich” ones, a white middle-class mummy who can afford to shop at the local farmers’ market. The reality is that I’ve been as tired, strapped for cash and busy as anyone else over the past year. But I’m fussy about food. I think my children deserve to eat well. If they’re hungry, they can help themselves without asking to fruit, toast and milk. They don’t have video-games. We bike to school. We’re so old-fashioned we’re almost hip.

Even my children realise how tricky it is to comment on what others eat. When I told my eight-year-old that I was planning to write about his friends’ lunchboxes, he looked pained. “Don’t you think that’s rather personal?” he asked.

Personal, yes. But political, too. We’re kidding ourselves if we deny that what others eat affects us, too. The boy in my son’s class who eats the two Mars bars and packet of crisps each day is also the most disruptive child in the class. I don’t need a scientific study to persuade me that diet affects behaviour — I’ve seen my boys after chocolate ice-cream.

It’s too easy to pass the buck. Everyone has had a hand in letting our children down. Yes, the big food companies are shameful for pitching Thomas the Tank Engine yoghurts at our toddlers. The muscled Williams sister who promotes McDonald’s in TV ads with the slogan “I’m Lovin’ It” should be made to play at Wimbledon with her serving arm tied behind her back. But no one is forcing us to feed junk to our kids. If you can’t or won’t cook, give them a banana. Scramble an egg.

HOW TO TEACH YOUR CHILDREN HEALTHY EATING

It’s not difficult to get most children to eat well if you set a good example, make mealtimes fun and refuse to let food become a battleground with fussy eaters. My friends used to laugh at me when my sons were babies, but I made all of their food. Whatever I cooked for my husband and me I served to them without salt as a purée. When possible, I ate with my toddlers, even if that meant eating a second dinner later, and discouraged the ordering of dumbed-down “kiddie meals” when we ate out. My boys would split an adult portion of “real” food instead of nuggets, baked beans and chips.

At home, we cook together. I’ve always been happiest in a kitchen, and we eat a variety of foods. I invent recipes for them, often just by giving a silly name to leftovers, and they go for it. I try to buy in season, and am fussy about quality for reasons of taste. You won’t get a child to love runner beans if they’re tough and taste of nothing. Mine will try many vegetables if there’s a good sauce for dipping, and they like colours.