WE'D been looking after Jonah for a couple of days last November, while his mum and dad had a break. We'd all had a lot of fun. We'd been to the zoo again, our third visit. This time he wanted most of all to see the toucans. As soon as he spotted one, he gave a sigh of pure delight and breathed: "This is my favourite in all the world!"

We'd fed the ducks and watched a bunch of trainee Horse Guards making rather a mess of their parade routines in Kensington Gardens. We'd paid a family visit to Jonah's aunt, our daughter, not far away. We'd read lots of stories and sat in many imaginary cars and eaten many imaginary meals (as well as a good many real ones, of course).

There were odd moments when there was the inevitable clash of wills - and the will of a two-turning-three-year-old is a strong and tenacious one. But Jonah's mostly a well-behaved child and didn't give us any real trouble.

The only thing was, we were staying in his house, and there were a few occasions when we weren't quite sure what his parents allowed him to do and what was definitely forbidden.

Take the sofa. It's a stylish sofa, nearly new and a pale oatmeal colour. Jonah wanted to throw all the cushions on the floor and jump up and down on it. We wouldn't have let him to do that in our house, so we said "no" when he wanted to do it in his.

A few hours later, his mum and dad were home. Within minutes the cushions were scattered on the floor and he was jumping up and down on the sofa, his daddy sitting calmly beside him. "Look, Nana!" he shouted. His face was a picture of triumph. It said as clearly as any words that he'd got one over on me.

It was then that I suddenly realised how much he'd grown up. He was no longer just a toddler, acting on instinct and impulse. He'd discovered how to manipulate the adults in his life.

Grandchildren are a huge source of joy for their grandparents. But that doesn't mean they are always a pleasure to be with.

Even our beloved grandson can, now and then, be less than perfectly behaved. Which brings up the vexed question of how to deal with these moments.

For the most part, like most children, he's at his most difficult when he's tired. But when your grandchild's running noisily about in some public place then even the most doting grandparent can sometimes feel like saying: "This child has nothing to do with me". Especially when it's one of those times when, if you were in charge, you're sure you'd have handled him rather differently. But since his parents are there, you keep quiet about it.

As I see it, grandparents should never do anything to undermine the authority of parents. Which isn't easy. Given half a chance children very quickly learn how to play off the adults in their lives one against another, just as Jonah did over the little matter of the sofa.

That wasn't important, of course. But when it comes to the big things, it's no good if one adult says no to something only for the child to go to someone else and be told that's fine and he can go ahead and do it.

It's even worse if the poor child has four adults all saying different things to him at once. Especially if he's in the middle of a tantrum.

So the wise grandparent keeps silent and leaves it to the parents.

Sometimes you can see the parent you once were in your son or daughter-in-law, and want to say: "Hang on minute! That's what I used to do. I now know there are better ways. Try this instead!"

But as I see it, the only time you ought to interfere or give advice is when you're asked to do so; or when you feel the child would otherwise be put in real danger. Nearly all the time, for nearly all grandparents, the only right response is silence. Just as you yourself found your own way as a parent, made your own mistakes, so now your own children have to be allowed to do the same with their children.

And, of course, it's always possible that they may find a much better way of doing things than you ever did.

But we're none of us perfect, and is there a grandmother alive who hasn't, now and then, spoken out when she should have held her tongue?