What is the ruling on a divorced woman concealing her second marriage from her ex-husband, who still provides for their children, with the approval of her new husband, who does not provide for them?

The divorced woman, after the completion of her iddah (waiting period), becomes a stranger to her ex-husband, and he has no right to know her affairs. However, it is not permissible for her to conceal her marriage in order to continue having custody of her children, because custody is forfeited by the mother if she marries a stranger to the child, based on the saying of the Prophet, peace and blessings be upon him: "You have more right to him as long as you do not marry." As for the maintenance of needy children, it is obligatory upon their father.