Don't try the iron on patch suggested by the Momma below. Leotards are synthetic, the iron would melt the fabric sooner than it would melt the glue to adhere the patch.
Try this.... turn the leotard inside-out and stretch it over something glass or plastic... ie: if it is a leotard, as in a pair of leggings/tights, and the hole is in the leg, put a drinking glass inside the leg to hold the fabric open. If it is a leotard as in the actual body suit, try turning it inside out and inserting one of your glass mixing bowls into the body section. The idea is to separate the fabric. Take some clear nail polish and very lightly "paint" the inside edge of the hole. this will keep it from running any further. Once the nail polish has dried, take a needle and matching thread and from the inside of the fabric do the best that you can to sew it. It will inevitably leave a little pucker in the fabric.
If you do want to go with the other momma's suggestion about a patch, buy the patch (they do have really cute little flowers, butterflies, etc.), and buy a bottle of fabric glue to adhere it with. Either that, or sew it on. Just don't use an iron on leotards or you'll have a bigger problem than you started out with!!! :-)
Good luck!