I wouldn't count on being able to find a bathing suit at WDW in December. When we were then in December, I was trying in vain to find a pair of shorts, since we had forgotten to pack DD's. I was told that Disney doesn't stock many summer things during the winter months, since is usually is colder in Dec and Jan. I asked in SEVERAL stores, and even had Poly concierge looking, nothing at all on WDW property. Luckily it didn't get that warm and they really weren't needed. I'd buy a 18 month suit when they go on sale. That should be big enough, but not too big to be usable. If your child is on the slim side, then buy 12 months, my DD who is 5 can still fit into size 3 suits since she's so slim, but needs 5 for everything else!!!