The same steps you take to make sure they are not smoking in a back alley on the way home.
Parents are not expected to follow the kid home from school and watch it every second. Parents are expected to have general talk about drugs.
Oh yeah, and it is illegal to maket smoking to kids. Meanwhile, facebook spends billions on A/B testing increasingly sophisticated addiction builders ... and you expect the kids and parents to just be able to win over it with no support.
Idealistic, I know, I know. But so is the idea that you can seal your kids in an airtight bubble and never risk any harm without the harm avoidance becoming harmful in itself. Or that the lawmakers in this country and elsewhere actually care about child safety at all, when they will not move a finger to prosecute those who hanged out with Epstein on his island.