It is indeed illegal in the United States to instruct a minor in any religion without the parent's consent. This would include instruction in any of the Pagan religions such as Wicca. You can read more about it at http://www.spellsofmagic.com/read_post.html?post=556828
If you're talking about Witchcraft as a religion the law would apply in that case as well.
However, there are no laws against teaching magic to anyone, even minors. So if your idea of witchcraft is strictly spellcraft and magic there are no such restrictions.