I don't know if any of these would be show-stoppers, but they're points to consider:
* Who counts as a "kid" for these purposes? I suspect 14-year-olds have more in common--as Wikipedia users--with 44-year-olds than with 8-year-olds?. If someone wants to set up a Wikipedia based on age/reading level, they need to define what that age and reading level are.
* It's a Wiki. That means *the users can edit it*. At a minimum, a kids' wiki is likely to be full of booger jokes and similar stuff that we'd immediately edit out on the English (and, I assume, the other language) Wikipedias. If this is really a kids' Wiki, can we-as-adults judge the proper content?
* I Am Not A Lawyer, but I believe there are real difficulties in having even our minimal-information registration for children under 13, since we're in the U.S. There's nothing in theory to prevent an entirely-anonymous Wiki, but I suspect it would change the environment.
Having thrown all this out, I'll note that I don't plan to work on this project, if it goes ahead, and my only *strong* feeling is that if it's going to happen, it should have its own defined namespace.