I think this is very disturbing. For one thing, the OP doesn't say whether the child is alone in the yard for hours, while he/she is screaming. If alone, there is defibitely something very wrong with the whole situation.
If the child is autistic, or developmentally disabled, and the grandparents are out there in the yard also, that's a different story. If that is the case, what can you observe and hear? Do they seem to be trying to calm the child down, are they playing with him, etc.? Or do they seem to be angry, threatening, punitive? They could be inflicting horrible punishments on this kid once they're in the house.
I would definitely talk to the grandparents and see what I can find out. Yes, kids do play and they do make noise, but not alone, and not constantly screaming for hours. I wouldn't even treat a pet dog that way, let alone a child!