Is this considered child abuse?

You are right it is a form of child abuse if the parents are feeding this kids unhealthy food. Some people are just so stuck in their own ways they don't realize what they are doing to themselves and the kids. I would pull them to the side and tell them or even start introducing healthy snacks to the kid when he is over your house. The parents might say thanks or they might tell you to mind your own business but at least you would have tried and there is a right way to address them. You can leave pamplets around for them to read or give them healthy information let them know you care. If that doesn't work find out if you can get someoneProfessionall involved to talk to them Good Luck!