Yes, this is a good time to cruise if you want fewer kids. People with children generally wait and sail with their kids over the holidays. In December the weather in the Caribbean is generally wonderful. It's always hot there, but the humidity is way down and the sort breeaes the Caribbean is famous for have started up. Great time of year to cruise and the fares are wonderful too.
We have cruised almost every Dec since we started in 96 and the weather has always been beautiful. There will be less kids as they usually wait until vacation time. A bonus is that often this is a real bargain season!