My daughter is at that in between size at 13. She has a little belly and does not like to show it. Sometimes she wears a 16/18, sometimes misses, sometimes womens. It just depends on the style and cut.
She needed a new suit for our trip last summer and we ended up finding a tankini at JCPenny didn't feel too too old. They sell seperates and we could buy the top a little bigger than the bottoms which worked out well. It covered her belly, had a slightly padded chest which looked more modest on a girl who is just starting to develop, and had adjustable straps (by pulling them all the way in, we had a great fit on top). We got a coordinating skirt-style bottom in a smaller size.
You can see that even with her arm all the way up, only a teeny tiny bit of skin showing at her belly.
I have to agree with other posters that tankini's are more flattering when you have a little bit of belly to conceal. Also a one-piece swimsuit is a pain in the tush when you need to use the bathroom. Tankini's are so much easier.
Good Luck. It's a hard age when it comes to clothes shopping.