Is it permissible to install child monitoring programs secretly or with their knowledge to protect them from harmful content? Is this considered espionage? What is the responsibility of those who sell these programs regarding how they are used?

It is not permissible to install programs to monitor the actions of children, adults, or wives without their knowledge, as this is a form of spying, which is forbidden by Islamic law. However, if the father informs his children that he has installed a program to monitor their actions, then it is permissible.

It is permissible to use programs to block bad websites, as this falls under the parents' responsibility to care for and protect their family members.

As for selling these programs, it is permissible as long as it is not known or highly probable that the buyer will use them for illicit purposes. If the buyer uses them for illicit purposes, the sin is solely upon him.