But seriously - if you think social media is harmful to children, I don't see why you would oppose making it illegal. It would be illegal, neglect, for example, to let your children wander into a busy street. Why should it be legal to let your children come to harm in other ways?
I'm not actually sure that social media is harmful, or how harmful, or what qualifies as social media or as "use" - but if we grant that it is harmful, it should be illegal. Doing otherwise just adds another advantage to children with good parents. Making it illegal will, hopefully, reduce the amount of harm done to those who do not have the maturity to decide things for themselves.