Call your ISP and ban any NSFW/NSFL access by DNS, both in your children's phones and your home connection. Problem solved.
Then they scrape together their pocket money and walk into a pawn shop and hand over the cash for a second hand smartphone. Plenty of free WiFi around.
This does not work, browsers like Firefox don't even always use the system DNS by default.
Then they scrape together their pocket money and walk into a pawn shop and hand over the cash for a second hand smartphone. Plenty of free WiFi around.