A lot of people don't know how to interact with children, specifically I'm talking kids younger than, say, 10, though this applies more broadly to whoever the person considers to "not be an adult". It isn't all that hard, though, just not obvious to many.
What you do is you treat them like everyone else. End of story. You don't talk down to them. You don't exercise authority over them. They are people, not things. Treat them like people.