I think giving them there own little patch and letting them design it, look at magazines etc, cut out pics and get them to glue onto a board, maybe with a theme. I think once kids start gardening they really enjoy it. Nice to take them out and about and buy seeds and plants etc. Even down to making their own bird boxes etc, somes good kids sites for this. My son made his own mini pond, a black bucket sunk into the ground with some slate and a few stones around the edge (left the handle on so could be easily pulled out if need be), he loves it, likes fishing about in it with a jam jar and seeing what he can find!