That would be two-factor, genius. Something you know and something you have.
Then make sure that the "something you have" stays at home. That way accessing their social media accounts would require a search warrant.
This would require cooperation from the kids not to create other accounts, etc, but for well behaved kids with a good relationship with parents, it might work until some time in High School, at which point you would hope they would have developed enough common sense and self control to avoid unintended inflammatory postings.