
The usual which springs to mind is radishess, carrots anything tasty which grows fast
I was thinking along the lines of peas (well runner beans really,but I am not sure if youngsters should work with them)
How about every kid decorating there own plant pot, emptying compot bin, sieving, filling, and plantng a pea seed (or plant so there is something to be seen). making a little trellis by bending both ends of a willow or hazel cane into the pot ...

Oh I don't know I think I am loosing it here
GOOD LUCK!!!!!!!