Debra,
I was there last December (first week) and the pool was packed! The weather was perfect!
Of course it can be hit or miss with the weather at that time of year, but I think it is reasonable to expect to get a few decent swim-worthy days. Just make sure you have some exciting alternatives planned for them, in case you can't, so that they are not disappointed. Think of hitting Downtown Disney to search for pirate loot!
An important pool note. Many of the pools are heated in the winter months, so even if it is right on the line of where you would consider swimming, keep that in mind. It might make the difference between heading to the pool and not.
Your pirates-in-training are going to LOVE the
Caribbean Beach Resort's newly refurbished pool. The castle complex is fun for exploration, and they can take turns pretending to battle each other through this well themed environment.
You better get them some eye-patches and sabres quickly!
Have a magical (aaaarrrrrgggghhhh!!!!!) vacation, matey!