I have some seeds of a really small sized tomato that you can grow in a hanging basket or 10" pot. It would be a good one for the kids because they are ready in 60 days and they live for a really long time.
The miniature sunflowers are fun and you can grow them in a 6" pot. Most of the kids won't be too thrilled with onions and radishes, it's not kid food! Banana plants are fun, not for the bananas but because they grow so fast and look different. Figs are also something fun to grow that grows fast.
Do you have supplemental light in your indoor greenhouse? if not, then it's going to be quite a challenge to grow anything other than houseplants.