Great idea, kids love to grow things.
My little one, who was 5 when we made her a raised bed of her own.
If the raised bed (2×4 is a good size to start with too) is going on top of the lawn, you don't need to build it that high. We used old scafolding planks (£1 each from a builder).
She started with a right mixture of stuff, peas, beans, sun flowers, strawberries, carrots etc, they all got flung in and some did better than her Dad plot 😉
There is a ongoing gardening post here btw:
http://www.singletrackworld.com/forum/topic/stw-garderners-and-allotment-users-friday-chat