First of all, I don't think a 9-year-old locking up the front wheel sounds like a good thing.
I don't have any experience with rotor necks, but I'd suggest closely observing all the parts of the braking system while you operate the brake to try to identify where the play is. Once you've identified that (there may be more than one place), see if there's a way to secure/tighten things to reduce the play.
In the meantime, ride with your daughter and help her to understand safe speeds and braking distances for the bike.