What is the age of children from whom a woman who wears the niqab must cover her face, and does this age reach fifteen years according to the Hijri or Gregorian calendar? Also, what are the parts of a woman's body mentioned in the Qur'an that children are permitted to see?

A woman's obligation to cover her face from non-mahram men is not limited to adults; it also includes those nearing puberty or who have become aware of women's affairs. Imam Ahmad mentioned that a woman should cover her face from a boy when he reaches ten years of age.

As for a woman's awrah ( Pudendum) in front of a discerning child who has no sexual desire, it is what is commonly visible to her mahrams. Anything beyond that is considered awrah. Some have said that her awrah before him is what is between the navel and the knee, but the first opinion is the one on which fatwa is issued.