That's very much my thoughts. The only way you really need to make a hard choice is if they take the child along. Can the life of an adventurer lead to the child getting "well adjusted"? Can the PCs keep her supervised enough to be safe while not getting her killed as an innocent bystander? Will they want to protect her if she continues her homicidal ways?
If they keep her with them, she's an easy go-to for drama - lost in a crowd, meeting random kids in town, kidnapped by the BBEG
Given your child-psych player, maybe add a supernatural element so if you go outside realism for the child, there's "plausible deniability"?