Hi I have 4 year old twin boys and honestly I thought they would show about 2 minutes interest and then give up, how wrong I was! they are useful diggers, not in a methodical way, but in a random haphazard way in order to find worms and "put them in their house" and sing songs about worms to them!, it has saved me a lot of digging! I just rake level, after liberating the worms of course!
I have planted radish (very quick growing) beans (I use runner beans they tend to be in the flower section in the US but can be eaten after cooking, Yum!)sprouting seed again quick. I am prepared to sacrifice some veg along the way to show them the growing process (tried to do it when thinning out but quite frankly I go with their agender, they will never want to know when you want them to! and they will want to pull them out! so use it as thinning process)
I used a litre plastic juice bottle cut about 3/4 way up and 3/4 round, roll up newspaper, quite a thick wodge and put it in the bottle, push runner beans (a couple will do in case of failure, never happened yet though) down between the paper and plastic to about half way down, put about 1/2 inch of water in the bottom, basically so it touches the paper for about 1/2 inch, the water will soak up the paper. seal the cut bottle with tape and leave the lid off, put on a window sill, with the beans facing into the room, and wait, it only takes a few days for the root to become visable and this caused great excitement, not just from them! at this point I attempted to cut a ungerminated bean seed...managed and the lack of a finger isn't such a handicap these days I'm told!!! my husband then said I should have used a peanut! it shows, easily the root and stalk parts! DOH! so do this not the bean! the root grows down and down to find food, but because it has a good food source the stalk will eventually also appear, at this point (which was today) you need to plant them although I am sure I have left them longer without ill effects! one of mine was so happy today wen I showed him, he hugged the bottle and said "thank you sooo much Bean" sweet I never tried but I'm sure other beans would work, anyone know?.
I also gave them a small part of the garden to be theirs and a piece of paper each to plan their garden, I printed out a sheet of pictures of veg (which I stole off on line garden garden catalogs)(use powerpoint if you have it to save paper) got them to cut out the pic and stick them on the paper limit 6, they made some interesting choices, obviously brocolli and carrot which they love, but cauliflower, which they hate, its for you mummy, no fruits such as strawberrys? and they wanted flowers again for mummy, I gave in and we are using sunflower and nasturtium, sunflower spectalcular, seeds attract bird which my two like and if any left after the birds! can be eaten, nasturtium, because my aunt assures me they grow quickly! I have never grow flowers before so an interesting experiment for me. I only intend to let them have say 6 carrots and hope they grow! because they really need ownership and responsibily at this age not quantity, thats my job!, be prepared to tend their plots when they get bored and above all enjoy it, I built some raised beds on a slope yesterday and was amazed how much I got done without them around but found my self bored because they weren't around to sing "there's a worm at the bottom of my Garden, and his name is Wiggley Woo" incidently I made the raise bed from an old toddler bed! and until my birthday last week I was using their yellow plastic watering can! check out making newspaper pots on the web, don't buy a shaper it easy, just damp the paper first, egg boxes for seed starting, I got hand tools from the dollar tree, they may not last long but they are a handly size for children. hope this helps

Thank you for your lengthy reply! I am encouraged by the fun you describe. :) I am looking at some premade raised beds I can order on line and just assemble to start. :)

Katherine

I did a container bean teepee for my kids (4 and 2) this year. I have photos and directions:
http://www.ehow.com/how_4836077_make-bean-teepee-kids-garden.html

Have fun!

Bean teepees are great! I want to try one this year with some pumpkins and gourds behind it :)