"Grandad, am I fat?" The sort of question to make you spring immediately to the defence of your grandchild. Surely he shouldn't be worrying about body image at six years old?

Yet the question derived not from playground meanness but from a school letter. He had been identified as mildly obese.

Obese.

Of course he's not obese. Is he? I mean, I write about food for goodness sake. I tell people about buying good fresh food, and encourage people to eat sensible diets. How can he be obese?

Then comes another question, an often-asked one in our house.

"Grandad, can I have a biscuit?"

The answer is immediate, and telling: "Of course you can". Because that is what grandparents say isn't it? - "of course you can". Because we are the grandparents and we have earned the right, to say yes. We struggled through all those years raising their parents. We had many times when we had to say no.

And our own parents spoiled our children didn't they? So this is our time to sit back and not have to worry about what they eat. Sure, we support what their parents do and say, but we are the grandparents: the ones who sneak a cake or a chocolate bar; the ones who say "don't tell your mum" with a conspiratorial wink at our shared naughtiness.

Obese.

Looking round the playground, on my school-run day, I compared my grandson to the other kids. He was like most of them. Some were bigger - fatter, I suppose - yet he is, what? Chunky is a good word.

Not unhealthily oversized. I don't think.

Yet, even as I try not to notice the roll of fat over his waistband I cannot ignore the news stories about the number of obese children in the UK, and the massive risks of stuffing young mouths with excessive amounts of sugary foods.

The chat among the mums and dads in the playground certainly features the 'fat letters' as they have become known, but lacks any sense that their kids' dietary health is a priority. No, that's not fair. They seem to just not have the headspace to make changes. When I think of my own kids' lives I can see how that happens: overtime working, or holding down two jobs to make ends meet leaves little time for major life change.

But as time marches relentlessly on how will our grandchildren avoid future health disasters (which our NHS seems increasingly ill-resourced to deal with)? I think it is time for grandparents to step up to the mark, relinquish our role as cosy provider of treats and ditch the biscuit tin. We need to learn to say no, but more than that, we need to be the ones actively seeking out better food for them.

Modern grandparents have the world at their fingertips, let's not condemn our grandkids to health problems that may mean they don't get to enjoy the lives that we have.

You read more about Steve on his website www.carefulfood.org.uk/, where he writes about food and food-related issues.