It’s easy to know what your children are doing with a mobile spy app . You will know what they talk in their chatting apps, what sites they have visited. Furthermore you can do something to protect their safety once anything wrong is found.
It's normal that kids block their parents from their social platforms. You need to give them privacy... How old is your son? If he is too young to use a cell phone independently, you can install some Facebook spy app on his phone. In this way, you can secretly check his Facebook chats and know whether everything is ok with him. BTW, if you really plan to do so, you'd better tell him what you are doing with his phone.