Parents should be more responsible. That's it. This measure is, potentially, deeply ingraining the (terrible) idea that the State is responsible instead, so when all these young kids have children, they, just as their parents, will lack the ability to take responsibility and make their children more responsible by proxy, and so on, and so forth. It's a never ending cycle that is perpetuated by not tackling the problem at its real source. And let's not forget how measures taken in the name of security are oftentimes actually made to deprive us of our privacy.
The difficulty is co-ordination. My job as a "responsible parent" is much more difficult if I have to fight prevailing social norms and my kids perceive they are being excluded from conversations and arbitrarily cut off from their peers.
The social media ban is similar to the logic behind gaming limits in China. The idea is that while the controls themselves are easily circumvented, it gives everyone an excuse to do the right thing.
Parents don't have infinite "control tokens". I only have time & energy to put my foot down about a limited number of things. It is much easier to establish conventions around responsible behaviour if the whole community is behind it.
I am OK with this ban for the same reason I'm OK with tobacco sellers being not allowed to sell to under 18s.
This, I would go so far as keeping kids from social media is in conflict with (arguably) one of the most important jobs parents have which is getting kids into social interactions. (E.g. by teaching them good manners so others will play with them)
How does a parent compete with trillion dollar corporations that hire psychologist, cognitive scientists, and neuroscientists to make their apps highly addictive?
Being honest here because just telling parents to deal with a societal ill seems very shortsighted and comes from an immense place of priviledge.