Gently counsel the supervising adult on how to string/unstrung about and keep the string on the bow! Also teach your niece how to do it. You might build her a stringing board as well. Just emphasize the she has to stand on the board while striking/unstringing. I learned the step through method more than 70 years ago, never knew about push/pull until after both shoulders were operated on. I use the board exclusively now, I outweigh the bow pull!
Hawkdancer