They just know that this person feels good and that person doesn't. And they have no qualms about letting you know where you stand with them. If they like you, you'll know it. If they don't, you'll know it. They don't worry about being polite and hurting your feelings.
Relationships are a simple matter for very young children.
If you feel good they want more of you. If you feel yucky, they want to get away from you as fast as possible. No guilt about their decisions and no ulterior motives. Maybe kids have more to teach us than we sometimes give them credit for. They see their role typically as straightening out the kids.