Well, that's kind of what social media is- people posting about what is going on in their lives. I could see it being obnoxious if the tone was all about rubbing it in someone else's face or putting down teammates, "My kid beat everyone on her team- AGAIN!" or something like that, but I don't get taking issue with a parent posting routines, or clips from a theater performance, or track meets etc. If I found it bothersome, I would snooze the person for a while, and I guess if it really upset me, I'd unfriend them as I have with people who I've discovered have social or political views that I find deeply troubling.
No, I don't watch every clip of a friend's kid's basketball games, or chorus concert, but I usually "like" them, and it doesn't bother me if they post those things regularly. There is so much in the world that is dark, and so much that is negative that I'm quite happy to share and see friends sharing joyful and positive moments.