What is the religious ruling on one who enters upon their parents during the times that Allah has forbidden, intending to do so, and what is the expiation for that, and will Allah forgive him?

If a person who is accountable [mukallaf] enters without permission, he has committed a sin for which he must repent, because an adult must seek permission, unless he is owned by the one he is entering upon, in which case it is sometimes recommended for him to seek permission. However, if the one who did this was a child, he does not sin because he is below the age of accountability, and seeking permission is desirable for him according to the majority of scholars, and his guardian should guide him.