almost anything can be grown in pots matey. Also, you don't have to do anything to your soil, except maybe weed it.
as mentioned, beans are a good option because stuff happens - they grow big and quick, and nice flowers, so kids get interested.
courgettes tend to produce a lot, so there's always things happening there.
you cannot beat the taste of a tomato you've grown!
herbs are good, as you can smell them (eg thyme,, basil etc) and can eat them fresh - pick some and pop it on a slad, or sandwich.