What should the cop have done?
Tased her.
Any parent of kids this age, knows that the more you demand them to do something they don't want to do, the less likely they are inclined to do it, if for no other reason than stubborn rebellion. By ignoring them and not giving them the attention they seek, the situation would have diffused itself on its own and a violent confrontation like the one on the video would not have taken place.
Just my opinion....