I am assuming you have a little bit of space outside? I really like peas and beans, especially with a youngster. Pea sprouts are really sturdy- about the thickness of a pencil- as opposed to something easily killed by a sprinkling of water, pesky cat, or ambitious toddler. They also germinate quickly.

You can't kill mint. It's practically a weed. Potatoes are also really, really easy and an exciting suprise at the end of the season.