What would possibly be the consequences for either parent or child if they violate?
If the law designed is any good, there should be none. The responsibility to check whether the parent has agreed should be laid on the social media giants, not the child or the parent. It should be a tool for parents to control the social media consumption of their kids. And after reading the article, which I highly recommend, it seems to be the case:
The broad law comes with heavy burdens for online platforms. It requires basically any digital services provider that collects an email at sign-up to conduct age verification to identify all minors, verify parents or guardians connected to all minors identified, and secure parental consent for a wide range of account activity.
Please read the articles that you post. Asking follow-up questions that are already answered is a little stupid.
Someone mentioned invoking GDPR's right to be forgotten. Although comments are not strictly personal information, it could still work. I think I'll try it soon.