According to the Terms of Service, you do. I think this is mostly to keep kids safe in case there is adult content that manages to weed it's way into hubpages. Another reason could be that adsense and whatnot requires a certain age before she can sign up. These are really just guesses on my part.
However, there are other places where she can place her writing. Schools often publish or share childrens writings, and so do magazines for the younger population. If you want you can help her set up a blog (livejournal or the likes) or a facebook page where she can share with her friends. Of course...I believe those places have age terms too, but they are hardly checked. Or you can help her build a portfolio for when she is of age. ^_^
Anyhow, even if she can't post here, there are other places more kid friendly. If she has anything written already, I'd be happy to read it. ^^
Hope this helps! Good luck to your daughter!