Unfortunately, this issue isn't treated with as much thoroughness as it should be in halakha. However, there is the concept that an abusive parent is violating lo titen miqshol lifnei iver and thus baiting the child into violence.
I know, and I apologize that I can't find the source, that the Tur argues with Rambam about whether or not a child is still obligated to honor a parent who is a rasha (variously defined). Rambam, and Maran Shulchan Arukh, and Rav Yosef all follow Rambam's opinion, but periodically posqim will reference the Tur's position on the issue in order to afford leniency to someone put in such a situation.
It's possible that you could also rule that the abusive parent is a rodef (providing there's real concern of fatal harm to the abused party) and use that paradigm to prevent criminal action against a child that harms their abusive parent.