As long as the pony is well-trained, I would go with a french-link. It is pretty mild and the double joint will help it fit in most mouths comfortably.
I would only use a kimberwick after the child knows how to ride - including having a secure seat and quiet hands. So if this child is a beginner it will be a ways off before they would be ready for that level of bit. It can be useful in the right situations, but a child just starting out is not it.