I don't think the online predators are the worst problem of posting pictures of other persons. (I mean, that is of course the worst case scenario, but not that probable.) Far worse is what the service provider or "internet" might do with the picture. Security settings don't really mean that much, as pictures are usually very easy to take from any site. I frequent a huge image sharing site and wince internally every time I see there a post with description "got this from my Facebook feed" or "my friend posted this" or other similar cases where said friend or friend's pet or kid or car or wossaname is published outside the original media. It's not the predators one should fear but oblivious friends. And then the picture is public for basically 7 billion people to use as they see fit. With luck, nothing happens and the picture just dusts there, or it can be next Bad Luck Brian. A good example of case, where picture itself is unoffensive, nothing bad or even funny in it, yet there are thousands of variations circulated around, and more come every day. And even that is the friendly way
then there is, of course, the unfriendly way of using loads of photoshop skills and making you lose your work over naked pictures of you (in reality "you, with other body").
In addition to that, of course there is a change that your son will be used as a billboard. Not sure if you meant Facebook, but I'm sure it's not the only one with something as below in ToS: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License).
It is a risk, and I'm fine with people taking it for themselves (as I do). But I think it is a risk one should not lightly take for other people. If the person being posted about is fine with it, then there is no problem (if they can understand the possible consequences).