Enforceability is a foregone conclusion, and when it comes to things like this it's somewhat expected. The same can be said for pornography, drugs and alcohol and tobacco (remember Joe Camel?), and anything else that would fall under blue-laws.

The goal of this is to bring attention to the fact that it's a problem and should be seen as undesirable, like pornography or Joe Camel. The cancellation of Joe didn't prevent kids from getting cigarettes but it did draw attention to the situation and there has been a marked decline in youth smoking since the late 90s when the mascot was removed. It's correlative, for sure, but the outcomes are undeniable. The same happened with the DARE program and class 1 drugs (except for marijuana iirc).

It doesn't have to be perfectly enforceable to have a positive effect.

Even just making illegal the promotion of social media toward children would have a huge effect.

This discussion can be seen every time when the EU decides on some regulation against tech industry. A lot of people will jump that it won't be enforced, then when we see the first fines those people will jump that it won't move a needle, then when the tech giants do change a bit their course then... well the tech bros will always find a reason to jump against doing anything to curb tech.