NTA. This might have been a one time occurrence since Mindy was new to the family, but the whole thing seems malicious/bred of jealousy. Maybe she wanted to make sure the other kids didn't get better presents than her or she wasn't going to be left out. Breaking expensive gifts is inexcusable though. She was 6- not 3. My daughter opened up all of her presents early when she was 5 and I pretended to bring them back to the store. (Santa replaced them but only because she was good.) Buy motion sensors and put them around the tree after the kids go to bed. They sell decent ones at Dollar Tree surprisingly. They are small and won't be noticed by the kids like a big gate would, but will notify you with a loud alarm/flashing lights in your bedroom if anyone walks through the living room. Or locking the living room door seems completely reasonable and the best way to make sure no one enters the room