My experience is, the younger the characters the more possible this is.

What I think is happening psychologically is the question, "How much do I have to lose by trusting this person?"

Children don't have much to lose by making friends, materially or socially. They often don't even understand how much they have to lose, eg kidnappers.

The ability to make friends is based around trust, and the more someone has to lose (or more specifically, how much they believe they can lose to another person), the harder it is to be friends.

This is why it is rare rich and poor are rarely friends: the rich person has too much to lose, socially or monetarily. The question keeps coming up, "Are they just here for my money?"