Why does the divorced mother's right to custody of her children lapse if she remarries, and at what age are children given the choice between their parents?

First: If the mother marries, her right to custody is forfeited, based on the Prophet's (peace be upon him) saying: "You have more right to him as long as you do not marry." The scholars are in agreement on this. The wisdom behind this is that marriage distracts the mother from caring for the child, and this may lead to the neglect of the child's rights and interests, because the husband may not be interested in raising a child who is not his own, which could harm the child.

Second: When a child reaches the age of seven, he is given the choice between his parents and stays with the one he chooses, provided that both are fit for custody. The child must be placed with the one who will protect him and manage his affairs well, even if the other party originally had a greater right to him; because the primary goal of custody is to protect the child and realize his interests, and this goal is lost if the child remains with someone who does not protect him from corruption and does not raise him properly.