In my biology class, we were put into small groups, and each group was given a few tadpoles. We then had to give our tadpoles a certain amount of some sort of hormone every day, put into their water (each group was assigned a certain level of hormone to give their frogs). I was in one of the groups that had hardly any hormone, so our frogs grew at a fairly normal pace, but the people who gave their frogs a lot of hormone -- it was amazing. Their tadpoles grew into frogs in a very short time period - but some of them died, or had abnormalities.
That might be a bit too heavy for a younger kid, though, especially if the frogs die. It was an interesting experiment, though, and not difficult -- we just had to keep them fed, and keep track of how much hormone we were giving them.