No, it's rude and patronizing to express appreciation and amazement merely because a person acted as a person SHOULD act in a public place. It doesn't become less rude simply because they're a child and have to put up with your amazement. Would you compliment a well-behaved middle-aged white man? No, but you would notify the FA if he got drunk and started to misbehave. Otherwise, you would mind your own business. You would not be amazed and impressed that he kept his pants on and the lampshade off his head.