How old does a child need to be to wear contact lenses?

It all depends... There is no defined age at which a child is allowed to start wearing contact lenses. Several factors come into play: is there a medical need, say in a young child who had cataract surgery; a child with a very high prescription in one eye that cannot handle glasses; or a mature individual who can strictly follow contact lens wear and care guidelines? If you feel he/she is ready, they probably are.
Depends. It really depends on the maturity level of the child. Improper lens care, poor hygiene, and overwear can lead to permanent vision loss. I would only allow under very close supervision and would not allow extended wear (sleeping in lenses).