Because they are children and at that age all they want is to play. They are innocent and being rich or poor doesn’t bother them, in fact they don’t know that they are poor or rich. But as they grow up and when they ask for something that their rich friend have, at that time your reply would be “it is costly” or “we can’t afford” or “we don’t have that much money”. And this now slowly makes the children know that they are poor.
Your second question was what makes children rich and what makes children poor? It is due to their parents. They are the one who who define the status of their children.