We have taken our kids to Costa Rica, Playa DC, PV, Hawaii, Spain, Disney
I think all are safe and fun. We have 5 young girls. They always talk about Costa Rica, but that was our most recent trip. We saw by far the most wildlife there, and we are into that. Belize is supposed to be nice and safe too. I don't think you can go wrong with any of them but it depends on budget and what you are in to.
Good luck.