I actually tweeted to Joe Belifiore asking whether Cortana would work within Kids Corner. Didn't get a reply.
I think jmshub is right about the legal issues related to minors, but I also wonder whether it could also have to do with voice recognition. Children's voices, especially very young ones, are quite different from adults, so you'd really need to train the voice recognition with voice data from chidlren, which I suspect no one has really done adequately yet.
It's a shame, really, because in many ways voice is the perfect user interaction modality for kids. Young children who can't read yet do fine with many touch and icon based interfaces-- they are happy to experiment touching various things and seeing what happens (to the chagrin of parents sometimes!), but as soon as they hit a dialog box, they come running back to you to help them figure out whether to hit the button with the "O" or the one with the "C"!
In the future, though, I can imagine a scenario where a child could talk to Cortana and ask her to tell them a story. When she can reply "Sure, honey", imagine the love that child will develop for this inanimate object.