Call your ISP and ban any NSFW/NSFL access by DNS, both in your children's phones and your home connection. Problem solved.
Please explain that too me.
I'm sorry for getting a little steamed here, but I have to wonder if you've put any thought into what you're asking for in the name of kids safety. And worse, if you think it will work globally what are you going to do when Saudi Arabia wants anything they don't like banned in the US, for example.