L.R.
Hi K.-
I used to be an early childhood special ed teacher and I have NEVER heard of strapping a child to any kind of chair, especially during a special event. I think that is terrible, and I feel so bad for your daughter and your family. I can't imagine how you must have felt watching that.
The only reason I could justify doing this would be if it were for safety. If there were a chance your daughter could fall out of her chair and become injured, then maybe, but otherwise not a chance.
I would talk to the director or principal of the school to see how this could be handled differently next time. Perhaps her aide or para could go on stage in all black and act as a helper for her. Remember, no one cares for your child like you do and no one will advocate for her the way you can. Good luck!