People are sending messages to others that include characters the PS4 can't read this "bricks" the PS4 meaning it breaks it and you have to factory reset the console losing all the data on the console. To stop this go to the messages tab on the PS4 itself -> press the options button -> privacy settings -> log in to the playstation account -> change your settings to receive messages from friends only or no one, depending on how much you trust your/ your kids playstation friends.