Is it permissible for a father to open his children’s private messages without their consent? Is reading such messages considered spying on them? Thanks.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

In principle, it is impermissible for a Muslim to spy on others, try to find out about other people's private lives, or to think badly of them. Allaah The Exalted Says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy…} [Quran 49:12]

Moreover, the Prophet, sallallaahu ‘alayhi wa sallam, said: “O assembly of those who have believed with their tongues but faith has not yet entered their hearts! Do not backbite the Muslims and do not search for their faults, for he who searches for the faults of others, Allaah Will Seek his faults, and whoever’s faults Allaah Seeks, He Will Disgrace and Expose him in his own home.” [Al-Bayhaqi: As-Shu‘ab] [Abu Ya‘la: Musnad]

Furthermore, Mu‘aawiyah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “If you seek the faults of Muslims, you will corrupt them, or you are about to corrupt them.”

This is the general rule, however, if the parent knows that his children are having illicit and forbidden relationships or the like, he will have the right to investigate it within the due limits of benefit so as to restrain and deter them from committing such wrongdoings. The parent in this case has equal rights to the one who is working in the Hisbah work, i.e. the authority concerned with enjoining good and forbidding evil. The latter has been allowed by Muslim scholars to search for and disclose sinners as indicated by the Encyclopedia of Islamic Jurisprudence; for the rule qualifying him to assume this work is to enjoin good and forbid evil.

Allaah Knows best.