go into the sprites house (eat the mushrooms outside it to get in) then talk to the little pot thingy about 100 times, just keep pressing x (it repeats each saying about 10times or so) eventually it will say somet like 'ok you win, what do you wish?' and you can choose to have a boy or a girl.
i chose a girl and i still got a boy, i dont know why that happened. Someone on this site has been able to get a girl though so if you want a girl try wishing for one and then don't speak to the pot again.