The girl was probably doing stuff that neither side is talking about
Anything is possible, but because she is only 8, I can’t think of anything earthshattering-or not correctable at home-she might have done. But teasing about the kid’s weight could well have provoked some inappropriate behavior in response, and we have no idea what the child might have told her grandparents if that happened. All of us who are parents here know how cruel children can be.