From Series 6 episode 8, "Let's Kill Hitler" (emphasis mine):
(Melody begins to regenerate.)
RORY: What the hell's going on?
DOCTOR: Back! Back! Back! Get back!
MELODY: Last time I did this, I ended up a toddler in the middle of New York.
But this is presumably the regeneration we see at the end of "Day of the Moon" (Series 6 episode 2), when she was a child before regenerating as well. Regenerating from a child to a child makes sense, but what about regenerating from an adult into a child?
Well, in Series 3 Episode 11, "Utopia", we also have this (emphasis mine):
MARTHA: Where did you get [the watch]?
YANA: Hmm? I was found with it.
MARTHA: What do you mean?
YANA: An orphan in the storm. I was a naked child found on the coast of the Silver Devastation. Abandoned, with only this.
So it seems like the Master must have regenerated from an adult's body into a child's at some point, before using the watch to forget his Time Lord heritage and then ageing naturally in a human body, just like the Doctor would have done in "The Family of Blood" if he'd stayed with Joan.