I think "relaxing" is subjective. I was going to suggest the Beach Club for it's "beachy" theme, but honestly it can be pretty hectic there. The pool is awesome, and has a sand bottom, but it is a busy pool. The area can be kind if laid back, but there is a lot going on, and usually has a lot of people around.
The Poly is a tropical beach, and again, depending on your kids and your ability to filter out the folks meandering around, I think this one is a good choice to hang back and relax.
The Grand is one of the most relaxing resorts, IMO.