That devolves into the age-old debate of restriction vs regulation. What devices should children have access to, and what devices are a problem? A laptop can just as well be used as badly as a cellphone in this context. Video games on home consoles can have just as much of a psychological effect as hate-mongering social media. I have had similar opinions (even reflected them in a past comment), but upon some feedback and introspection I've come to realize that forbidding kids from accessing something as ubiquitous as cellphones can't be viable. There is no set standard where you can draw a line that an individual is mature enough to handle social media, and there's no set consensus about when and how harmful it is that it would warrant bans. A handful of researches and surveys show that a section of children may face self-esteem issues from Instagram, but that's really not much to go on.
btw, I share the same bewilderment for children participating unrestricted in social media. I just don't have an answer to a practical standard of restriction. It seems like this generation's children will grow up having social media as a natural, integral part of society. So what it should boil down to, in my opinion, is to make social media spaces as safe, civil and habitable with laws, as you make the open world that children are exposed to. Regulate these platforms stringently, with constant supervision from public bodies, just as the police monitors real-life public spaces.