This is actually a big issue for me too. I generally don't say anything, because I hate confrontation, and know it won't go over well. "What do I know?" as you say (since we don't have kids).
My husband is a lot more outspoken, and has gotten into some trouble with his sisters over it. He has a hard time not speaking his truth when he sees them feed their kids junk food 3 meals a day, and let the TV raise their kids. He bites his tongue a lot, but sometimes it comes out.
One time we were having a family get together in honor of a visiting uncle from far away. Our niece wanted to turn on the TV, and my DH said, "No. We are not turning on the TV." His sister glared at him, and they have pulled away from us for that and other of his transgressions. Sigh.
He has other sisters luckily whose children don't watch TV constantly, and for some reason we can relate to them a lot better. It's actually like talking with a human.
I don't know what is best, but certainly there is a line that is crossed where you have to say something if the parents are just totally out of it.