Is it permissible for the mother to renege on her agreement with her ex-husband regarding the transfer of the children's custody to their maternal grandmother (the mother's mother) after her remarriage, and does the father's right to custody then fall?

If the mother remarries, her right to custody is forfeited and transfers to her mother (the child's grandmother), except according to the Hanbalis, where the father takes precedence over the maternal grandmother. A condition for custody to transfer to the father is that he has a woman with him to undertake the custody. It is not permissible for the mother to conceal her marriage to avoid losing her custody. In case of dispute regarding custody, the matter is referred to the Shariah court.