First recognize that you have shifted the question from an adult to a child. Even Israel had Judges to deal with tough questions. I think it's entirely appropriate to be prosecute a 14 year old child in juvenile court, if he practices witchcraft in a jurisdiction where it is illegal. I think how you ajudicate children is different than how you ajudicate adults and always has been even in early Israel. Depending on the situation there are many alternatives available to the judge.
But God's moral law says we have to kill her once she turns 18?