The main risk is that when you post, you are aware of the present context and can't link it to the future-- because the future doesn't exist yet. But computers can. Computer software can link a person's identity, the image of their face or name to every concept (website, blogpost tag, other people, ideas they express, actions they take, etc) they are seen with past or present. Facial recognition software can link all the pictures of a person to each other, and to the concepts they are linked to.
Computers are great at search, and great at storing links between information. Computers can make concept webs that link ideas, faces, names. Here's one example of a man who realized the story of his life could be drawn with pure data he wasn't trying to tell his story with.
Even if you are careful to never post your picture with your name, if one of your friends does once, the pictures can be linked with the names.
So, in summary, the risk is that you're telling a story that you can't see as you tell it. You're creating connections that allow agencies access to information that you or your child may not want them to have in the future. The fine print about the sites' "privacy" policy also changes over time. Even so, most sites maintain the right to share your data with agencies that your (future) child may not consent to.
This makes the risks hard to see, because we can't see the links between the data. But your child's future employers, partners, friends, and coworkers may have access to them.