Is it permissible for a woman observing her idda (waiting period) to uncover her face in front of children under ten years of age?

A woman is permitted to uncover her face in front of a boy who has not reached the age of ten, whether she is in her 'iddah (waiting period after a husband's death) or not. This is because there is no obligation to conceal oneself from him. If he is discerning (has some understanding), he may look at what is above the navel and below the knee. As for an adolescent boy who is close to puberty (typically around fifteen years old), it is obligatory to veil oneself from him just as one would from an adult.