Derpy is the only young mare with a child. I don't know why they put Dinky Doo into the show. In the Cutie Pox episode or whatever it was they put a smaller pony with derped eyes, was that a joke about Derpy having a sister maybe at first. But Derpy being a retard who gets taken advantage of by a stallion is funny to me. And her being tricked and easily persuaded into all sorts of things. Same reason she'd be so great with Trixie, well you can put that in there to.