Fun virtual pets like little dogs and cats are a perfect friend for kids. It instills a sense of responsibility, a feeling of caring for something that they find endearing.
When I was growing up, there was an experiment all ninth graders had to participate in — caring for an egg for a week as if it were a real baby. It made everyone realize how much time it takes (on a much smaller scale, of course) to care for something that depends on you for every need.
I think the virtual pets fill that same void.