Keep in mind that invading their privacy means breaking their trust at an age where a large part of their emotional and social development all but requires it. It's a good way to ensure that they won't come to you with any delicate topics later on, like questions and concerns about drugs, sex, or shady behaviour of peers — you know, the part where you want to be that trustworthy parent that they can talk to, always. To them this is like requiring them to use the bathroom with the door open with you looking in — you don't do that after a certain age unless you have a severely mentally handicapped child.

Also, you won't really block them from anything unless you forbid them to meet up with friends with their own unmonitored devices.