What is the ruling on a woman wearing short trousers that reveal her thighs in front of her sons?

If the children are infants (below the age of discernment), there is no harm for the mother to wear short clothes that reveal some of her 'awrah (parts of the body that must be covered); because the presence or absence of a non-discerning child is the same. However, if they are discerning, it is not permissible for her to appear before them in clothes that reveal her 'awrah, such as the thigh. As for adult children, it is not permissible for her to show anything of her body in front of them except what is customarily uncovered, such as the hair, neck, and the extremities of the hands and feet.