Answer: According to Piaget the mother has committed an error which is called "EGOCENTRISM".
Explanation: Egocentrism among children is the way a child behalves and think, that his/her thought is the same with everyother person, and they should act the way he/she thinks. it is the child's inability to see a situation from another person's point of view. According to Piaget, children wants everyone to see them the same way their see themselves.
The mother has committed an error because the little brother will want an equal share with the brother, even though he was crying, and this may cause the little brother to start crying, because he wants an equal share of the biscuits. The little brother may not understand the reason why the elder brother collected a larger share of the biscuits, because he wants everyone to see him with preference.