Two piece tankinis are very fashionable right now. They have bottoms that are not cut as high as most bikinis and a top that covers most of the midriff. Boy cut shorts are also popular, with either a bikini or tankini top. A lot of styles are mixed and matched now, so you can buy multiple pieces.
Teens like two-piece's, with optional wraps/covers for the bottom. However, one-piece's are coming back, with cute cut-outs on side or belly, backless(except strap), cute prints, and are typically figure-flattering.