simple girl, a few more tasks for your niece, if no one has advised her of this...
The week before school starts, she needs to go to each school her kids go to. To have the school records changed, so the husband is delisted as a contact person, valid pick-up person, and as a person who has the right to withdraw the student(s). She needs to talk with the proper office person, and must bring the court order with her. They will need to see the court order, and they might make a copy of it. No court order, nothing happens. This is standard operating practice in schools today.
Some schools keep a hot list of persona non grata handy at the front desk, most require the school staff to always check the computer before anyone can pick them up, withdraw them, see them, even bring them lunch. A lot of bad stuff goes on today among adults, and kids get yanked around or disappear with one parent/x-friend/doper/mental case/abuser/all of the above.
She should not wait until classes start, because the first day or two of school is a poor time to try to get things done, they have their hands full then. But she needs to work with the regular office people who handle this type of thing during the year, which usually means waiting till the week before school starts so they are all back and in place.
Once again, don't forget the court order. Otherwise, it is just another parent telling stories. No need to get into stories with the school folks. A few words and the court order will do it. They've seen it all before.