I think they probably did the right thing. They would probably have less physical contact with her by handcuffing her than if they tried to restrain her by using their hands until they could get her to calm down. I've been a pre-school teacher, and when you get a kid who doesn't respect authority and they start throwing a fit and fighting and flailing around, it is hard to protect their dignity AND stop them from hurting someone. This girl knocked over a bookcase, for one. Sounds like she hasn't gotten the right kind of training at home to control her anger, or there is something mentally or emotionally wrong with her that she can't control herself at 6 years of age. Such a sad story.
Whatever you do, work at it with all your heart, as working for the Lord.