You could just write the simple effect of the law: "EUR02a. Facebook employees read your private messages. If they see child pornography, they will call the police."
It's important not to phrase it as "read your messages to detect child porn..." because that implies they won't do anything else with the messages, and Europe is a place where they still assume that if someone says they do X to do Y, they're only allowed to do Y and severely punished for doing anything else.
It's important not to phrase it as "read your messages to detect child porn..." because that implies they won't do anything else with the messages, and Europe is a place where they still assume that if someone says they do X to do Y, they're only allowed to do Y and severely punished for doing anything else.