It can be a very good way to travel with kids and not have to pack and unpack all the time, while seeing new places. I think Disney and Carnival will take kids as young as two in the kids club and you can leave them there during the day if you want to go do an excursion.
A downside is that if you try to eat in the formal dining room with kids, it's not easy. We spent most of the time eating in the less formal areas.