I like January for your crew, depending on when. You know your kids better than us. Are they going to need a swim or a nap? Mine would have needed a nap at that age--not a swim. Add--the weather in January is way, way better for younger kids IMO.
In early January, yes, that is probably a little chilly, but I don't think you'd have a problem in later January. We were there Presidents Day weekend (which of course is February, but not far off) last year. We were in the pool every day.