Very small amounts of MDMA (or any other drug you're on) will be excreted in your sweat (they make drug tests that use sweat) but the amounts will be so tiny that it couldn't possibly affect somebody else even if they did ingest it. (MDMA is not well absorbed through the skin; you'd have to be kissing or licking them to get even that trivially small amount.)
So, it's perfectly safe to pick up your baby after using (the main risk of picking them up while actually high being that you might accidentally drop them.) As Bilz0r says, breastfeeding should be avoided for several days.