There has to be history still attached to the child if it is not rotating with the parent. If you cannot identify and delete the history node(s). Then your next option is separate the parts and duplicate the child. Delete the history again (to be safe). Delete the original child. Then redo the parenting. If you get double transforms now then you have something really borked.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675