Why not do the same thing, but maybe a little more advanced? The little kids could grow something like beans or melons, while the older kids could do tomatoes or herbs, or maybe some companion planting...my nephews are 10 and 13 and they're very interested in how plants benefit each other and which ones attract beneficial insects to the yard :)

Thanks for the ideas.......hydroponics is something totally out of the ordinary and I wouldn't have thought of it.