Personally, I think it's silly to put a pic of your kid or another person as your avatar. (on facebook). A few years ago a good friend passed away and many of my fb friends knew her, and a lot of them put her pic as their avatar. It was weird. I switch between my own (recent) pic and things in nature.

I suppose when they use pics of kids or grandkids they are showing them off. I don't have a problem with that, as I like to show my own off too, but my avatar represents me, not someone else.