where do you live? I ask because we have a program here in Peel (outside toronto) that you can refer parents to for hearing BUT you can call ahead and explain your concerns and they will take it from there. As a provider you can't tell parents there is something not right with their child but the first step is to always check hearing. The other things you described concern me as I used to work at a Deaf daycare and the children are typically up to par with their peers with exception of hearing. I have also had a child who did EXACTLY like you said I would go behind him and ROAR really loud or clap my hand and Nothing, no response. He is a hearing child and turns out he has ASD. All you can do is provide the best care you can and perhaps make sure you sit in front of him and have him look at you when you're doing an activity. Suggest to the parents that you notice he isn't responding when you call and that you will keep an eye on it as he may need some time to adjust . at least you've planted the seed. then as time passes if it doesn't improve you can more seriously tell them he needs to see a doctor to get his hearing checked