I think a 'children's wikipedia' is an EXCELLENT idea, but it would have to be much more closely monitored and supervised than the general one, which would probably remove it from the 'wikipedia' spectrum entirely. You can't create an online project for kids and not supervise it for appropriate content. Nobody would use it.
Something like this would really be a project for schools... I could imagine kids researching topics and creating articles as part of their general studies or computer literacy classes, and then uploading them as part of their class work. But rather than having each individual child have a login you'd want to set it up so that the TEACHER was the one registered... and most primary/elementary school teachers really think they've got too much on their plates already to do something like that. They're either not computer-literate enough themselves, or not interested, or too busy... so I doubt that the project would be convincingly taken up.