The WiFi policy seems good. But the invasion of privacy will only alienate you from your kids because you show them you don't trust them. (You shouldn't! But you shouldn't let them know that.)
I watched some dank ass horror shit when the internet was young. Tub girl, one guy one jar, beheadings, all that kinda shit. I was just a teenager. I'm a highly functioning member of society. Good work, good pay, wonderful family. You teach your kids what's right and what's wrong and they'll go by that. But you gotta LIVE it too. Don't just preach it. Show them what's right.
My mother trusted me completely to do the right thing. She shouldn't have. But kids don't always do the right thing. But me knowing I had betrayed her trust a few times made me feel awful and I learned that it wasn't worth it. Doing what's right is cool, in the end. Kids figure this out if you show them the way.
And if they never learn what's right, they weren't going to no matter what you did, because it will have been outside influence that was too great, and that's just destiny, you could call it. Nothing you could've done.
Best of luck to anyone reading this.