What an absurd, ignorant notion. Of course social media has a negative impact on developing minds, but forcing sites to display warnings would have zero positive impact. Browser extensions would immediately pop up to hide those warnings, and if anything, the presence of such warnings would increase kids' use of social media, since the danger is something even adults had a hard time understanding and kids love to rebel against oppressive systems. The warnings would turn into memes.
The only answers to this problem are to break up and ban social media companies (not possible) or get parents to actually be parents and teach their kids about the pitfalls of social media.