See if you can find Sharon Lovejoy's book "Roots, Shoots, Buckets and Boots". There are a dozen great kids' garden ideas in this book, from Sunflower Houses to Pizza Gardens (shaped like slices of pizza, growing pizza ingredients). See what your kids want to do!

My kids' favourite veggies to grow are radishes (speedy!), carrots (secret underground treasure!), lettuce ("picking salad" every night) and snap peas (for snacking on while working). Strawberries are always a hit too.

Miranda

My kids are a little younger (ranging from 1-8, including the close cousins) than yours, but we are starting a garden now with their homeschooling. Some ideas I have found are:

a sunflower fort, using those tall sunflowers just planted in a square big enough to make a fort- plant, water, and wait!

Teepee using bamboo sticks spread out enough to make a teepee and tied together at the top and plant pole beans on them.

Make sure to let them grow foods they will eat. My daughter will only eat cherry tomatoes- not big ones- so she grows them.

Plant a salsa garden with a roma tomato plant, jalapeno plant, and some cilantro. just add onion and you can have homegrown salsa.

Or a spaghetti garden with a roma tomato plant, oregano, basil, pepper- whatever you use if you were going to make homemade sauce.

It is also fun to plant interesting herbs- like chocolate mint, pineapple sage, or lemon basil- my kids think it is cool to smell chocolate from a plant!!

Check out the book Kids Garden by Avery Hart and Paul Mantell for some fun ideas.

Some sources to check out:
1.Cornell University's Garden Mosaics-all kinds of lessons on garden topics for kids
2.Kidsgardening.com-newsletter,ideas, books etc.
3.Jane Goodall Institute- Roots to shoots program for example right now they have little spring journals to fill out...more about nature than gardening really.

I see people mentioned the bean tepees and sunflower fort but another fun one is the potato tire stack. Fill a tire with dirt and plant a potato..when the bush is about 12inches high add another tire on top and fill with soil making sure there is still part of the plant exposed on top..continue to do this. When it is time to harvest just pull a tire off one at a time and harvest the potatoes that have been growing along. IT is very fun, takes up little space, and also you are sure to harvest all your potatoes (sometimes I don't find them all when I dig them up normally).