The framing I used is me trying to summarize in simplified terms for folks who aren't particularly legally savvy. In the USA minors have a more limited set of rights than adults (medical agency is a really solid and easily understood example for most people), which it sounds like you already know (I'm speaking for the crowd).

To answer your question, I'm really not sure. I think social media is 90%+ toxic and has overall been awful for society, and it's terrible for children. Absolutely awful for their development and self esteem. But that doesn't mean I think that preventing them from accessing social media is correct either. Even if I did, Mississippi's law is a garbage and completely ineffective way to do it and I wouldn't support the law as it stands even if I was dead set that kids shouldn't be on social media at all.

Edit: I didn't answer your question thoroughly. I'm not sure if it's unconstitutional to bar children from participating in social media. As I stated, I think it's harmful in general as it exists today. 2nd amendment is a constitutional right, but we don't allow 12 year olds to purchase guns, so there's plainly lines that we draw with minors. Does restricting access infringe on 1st amendment rights? Sure, but is there enough harm from social media access to minors that it's justifiable to find it constitutional to bar it? I honestly don't even know if there's a credible body of research to say for sure, and my anecdotal opinion totally doesn't matter. We need good evidence and I'm not sure if that exists.