It should be clear that mother has the right to foster baby boy for 7 years and to foster girl till she reaches adult hood, however, in case where mother marries a person who is not a merciful mehram then mother cannot retain the right to foster but maternal grandmother will foster the children and father will bear the living expenses. After the boy reaches 7 years of age and girl reaches adulthood, father will have the right to foster so that they get good coaching.
Allah knows best
Darulifta Al Ikhlas, Karachi