I don't know if it's necessary to "prepare" a child to wear glasses or contacts. I wore glasses at the age of 8 years old and there was no reason to prepare me. I could either see or walk into walls. Children understand that concept. If the child has any problems they have to realize that in order to see they need some help. There is no shame in it and if a parent makes too big a deal out of it the child might think that it is something to be embarrassed about.