It's not just the kids, the adults are quiet also. The first time I went to Germany I couldn't believe how quiet the airport was when I landed. It's like a library in America.
It's like someone shouting in a library, whether it's my American colleague asking sometime else in the office a question, a couple of tourists on the metro, or an American family in a restaurant.
It's probably a large part of why Americans are often considered rude.