[bolded by me]
I've heard of, occasionally, policies or local laws requiring victim involvement for criminal case decisions--these are typically for cases involving domestic/intimate partner violence (eg. the state cannot prosecute an abuser without participation from a complaining witness/victim). But by and large, it is just a courtesy for the state to get victim/family approval for any actions.
In the US, it is basically (generalizing here) like this: In a criminal case, the defendant is accused of breaking a law, and the state is the "injured" party for that violation of law, which is why the cases are named something like The People of West Virginia vs. the Defendant. It's a courtesy to have a participating and satisfied victim, and it makes things easier for the prosecution, but it is not a requirement. A civil case is where the victim can directly seek restitution as the injured party.
In practice, I think the prosecution always wants the victim/family to be happy with their actions, but I can think of two factors here that might affect that. First, this case has become very public, and so there will be political, etc. repercussions for whatever the outcome is. Second, there is a possibly complicated family situation to try and consider, since the victim's parents may not be on the same page and the custodial parent (JS) is suspected by some to have been involved in the crime.