I think the moods of adults can't change as quickly as kids. If someone is thinking about something with a serious look on their face and you flash them a big g

I think the moods of adults can't change as quickly as kids. If someone is thinking about something with a serious look on their face and you flash them a big grin it creates something of a cognitive dissonance since their mood is at a certain point in emotional space and you just generated some emotion too far from the initial point. To stop this dissonance from forming you have to lead them along from point a to point b one step at a time. Since the mood of a child can change so quickly you can just make a silly face and their mood will be pulled along to the emotion you just caused them to have. Unless of course, you make a silly face at a kid who is about to cry, that's a sure fire way to annoy them. I think you can also see this difference in the way kids and adults make small talk. "Nice weather we're having" versus "I love pokemon what's your favorite pokemon".