There is a part where you say child A dropped the stick on her legs. I assumed you probably meant B or C.

IMO, (and remember this is my opinion from just reading your OP, I did not see the situation), if I called the parents every time a situation like this arose, it could potentially mean I was calling a parent every week! I know it probably scared you more because it was a tree branch but kids could drop toys(and some toys are heavy!) on each other as well to be mean. Would you call for pick up then?

What I'm trying to say is....did it bother you more because it was tree branch(which is a bit more surprising to see) or because they were hurting a child?