S.
Regardless of what you say about the parents, it has to be traced back to them - you certainly can't blame the schools! :)
I don't want to sound harsh, but really, these children should be in a more structured setting even within their home. I have plenty of friends who homeschool, and their children are not out of control. There are parents who say they homeschool when really, they are too lazy to conform to the regimen of a school's schedule and expectations, and they are neither teaching their children subject matters nor social manners. Having broken toys, torn wallpaper, mice and insects in bedrooms full of uneaten food - this is just completely ridiculous and indicates much neglect on the parents' part. I am sorry your grandchildren are living like this, and I hope you will find the strength to intervene at this point. The parents need someone to lovingly but very firmly make it clear that this is extremely unusual and not acceptable for normal people. Children need boundaries and accountability, and it sounds like they don't have any.