It's not going to protect them from being pressured for nudes. It's not going to stop them from committing felonies and producing and transmitting underage pornography. It's not going to stop rumors and gossip and drama from spreading across Facebook even if your router is off for the night.
My brother is a middle school math teacher. Find the nearest middle school teacher you know and ask them how pervasive these issues are. Just from their small middle school, they had dozens and dozens of students disciplined, just for nudes. One was supposedly going to be charged after the 6th+ time of soliciting nudes from a fellow student and then sharing them. The quiet nerdy kid built a categorized Dropbox collection from the entire school and was quietly sharing it. It means so little to them.
I'm actually sort of getting worked up here thinking about this. If you've sat on your hands until your kid is 12, you have no trust or dialog with them, god bless your hearts for thinking a firewall or Android app is going to help a single ounce. All you've done is setup a tiny puzzle. Most of us likely know the workarounds, they will figure it out, just like I did 15 years ago.
Of course, I think "tech" is probably an invaluable tool for setting time limits and helping with screen addiction, but it's not going to fix any of the other psychologically damaging aspects of youth+tech.
EDIT: I really appreciate and would like to echo jedberg and ziddoap's comments here as well.