I had a look at the US Olympic Team's website for Women's Gymnastics to get an idea of what workout styles were in use for children on up.  This basic swimsuit has a fit that allows for athletic movement without binding or slipping, yet still has the bright color and sparkly trim an aspiring gymnast (18-month old) would want.

Our daughter wore this suit in 2T with pale pink yoga pants on top and bare feet for her first gymnastics class last weekend.  The combo worked perfectly and looked great.  The suit was a tiny bit loose, which I wanted.  (She's 40th percentile weight for length.)

If purchasing for swimming purposes, bear in mind that the diaper may show a little on those still of diaper age.  :)