Unfortunately, kids "helping" in the garden are usually very unhelpful. Here are some tips to keep them busy:
Give them a small watering can or a spray bottle. It's hard to ruin anything by watering it, and it might actually help.
Grow lots of things to eat. Berries are ideal, snap peas and cherry tomatoes are also very good, and of course something like potatoes won't work at all. But it doesn't matter too much what you grow; I've watched three kids help themselves to chives for an hour.
Make sure they have plenty to keep them occupied while you're doing your gardening. A large container or a section of the garden that they plant and tend themselves can keep them from destroying important plants. You can always put new plants in while they're not looking, after they kill the originals.
Of course, they don't have to garden. A chair to sit on, some bubbles to blow, sidewalk chalk, and a sandbox or pool are all good ways to keep a child occupied outside for a few minutes, and hopefully more.
And the easiest way to garden with your kids is to get someone else to watch them while you get some real gardening done.