I was at
Castaway Cay about this same time on our last cruise and at Key West in Jan two years ago. It isn't hot, but kids don't seem to mind the water!! I get cold easily and wore a long-sleeved cover-up and wrapped a towel on my legs. It was still very nice - in the low-mid 70s, but breezy. My kids will get in the water with chattering teeth and turn blue and not seem to care, so they will have fun!!!

if your kids swim in the lakes in Michigan they will be fine - it's all realtive!!! It can also be pretty cool at Cape Canaveral in Jan (or pretty warm - you reallly never know). My in-laws live in Orlando, and we have been there in Dec/Jan/Feb and worn shorts sometimes and worn heavy jackets other times. The coast is a little more mild. I would definately plan on a light jacket or long-sleeved t-shirt in case it isn't warm.
My DS (8) is going to enjoy meeting yours! My husband and in-laws who we are travelling with are also from Michigan originally - although no one lives there now!