Yes I'm looking for essentially a half-toothless law with punishment "if reported" like it is for the laws against leaving your child alone. Someone has to report you. If you're not reported, you aren't punished. Yet everyone I know complies with very few occasional exceptions, even if all of them think it's a stupid law. Even half-toothless laws can change the overall situation. All I need is the majority of her peers to have been forbidden social media. I can do the rest. If everyone she knows is on social... it's my word against the world and while I might be able to pull off argument that sticks, it'll be difficult, and the next guy might fail.

You don't have to false dichotomy it. There's other options than prison. The gov't determines what's social media. There can be diff criteria. The determination can be made by a regulatory body similar to the CRTC that updates what counts as social media now and then.