You say the magic words, and suddenly you both start glowing and you feel like you're being sucked out of your body, you close your eyes in agony and when you reopen them you are looking at...yourself. Or at least who you used to be, as you look at the baby sitting on the floor in a diaper.
If that is not you...then who are you? That question is quickly answered by a look down as you see yourself sitting on the potty, with the training pants on and looking a bit older. Briefly feeling inside your pants it quickly dawns on you - you and the girl have swapped bodies, and you are now her! Although this predicament is particularly lousy, you at least got one thing out of this: you also gained her memories, which should help you survive for now, and act like you are really her - but you better find a way to reverse all this real quickly, or you won't even have to act like you are her anymore because you ARE her.