This is going to sound mean but I would probably hold her nose and say, let me help you, some air might get in through your nose. I bet that would shock her and teach her a lesson. It's time somebody calls her on her ridiculous behaviour and shows her that she isn't upsetting you at all and she isn't going to accomplish anything with that trick. She isn't going to faint and she is going to breathe again when she has to, but obviously her parents are panicking and she knows it works on them. I am so glad I don't have school aged children in my daycare.