From what I understand, the rede is, more or less, telling you what IS good to do, not what ISN'T. An example would be that is it telling you that it IS okay to do magic that is beneficial, and it IS okay to be gay, and it IS okay to have sex out of wedlock, and it IS okay to get tattos, because these things don't hurt anyone. It does NOT say that things counter to this are not okay.The rede says nothing about what NOT to do, only what you CAN do without worrying about weight on your conscience.
The thought that the Rede speaks against negative/harmful workings is one of the base human assumptions that if something says that one thing is okay, then the counter must not be. it is okay to not punch someone in the face, therefore, it is not okay to punch them. It is okay to punch someone in the face to stop a rape, it is not okay to allow the rape to happen, so that you cause no harm. It is a side effect of mankind's tendency to polarize behind issues, making everything an either/or, instead of an as-well-as. We are naturally exclusive... inclusivity is a rare thing indeed.
The Wiccan Rede isn't a law and Wiccans are free to obey it or disobey it.
A Rede is a suggested guide for original action.
It instructs us to consider the greater good before we act on anything.
So in your ethics question, what is best for the child outweighs what is best for the abusive parents.
Gerald Gardner said,
"You must not use magic for anything which will cause harm to anyone, and if, to prevent a greater wrong being done, you must discommode someone, you must do it only in a way which will abate the harm."
This is basically a self defense clause that allows a Wiccan to cause the least amount of harm when attacking in order to reduce the overall amount of harm.
If the parents hurt the child?
So slap the parents down, have the parents arrested. Shelter the child. Allow the parents to see the child during prison visitation hours. EVERYBODY IS HAPPY. He, he.