That is a great question. You're right. The simple fact is that there is no consent for very young children. At the very least, there is maybe assent in that you know that the child is not doing it truly against their wishes, but they can't consent to all the implications that come with it.
To be perfectly honest, this is one of my arguments toward the fact that there is a date, probably somewhere around the age of 14 to 16, when young people can take back control of their digital identities, but before that, I do not see the benefits of allowing children to post and feature in social media content to a wide audience. Two things enhance the risk here: the amount of time they spend making content and, more importantly for what you're pointing to, the amount of exposure they get. You get past a thousand people. You get up to the millions and tens of millions and hundreds of millions of exposures. You are magnifying the risk infinitely, and no child has the capacity to understand what they're consenting to when you're talking about those large numbers.
My own six-year-old struggles with the difference between five and five million, so I can't imagine too many other kids really understand how big the exposure is.