What a fucking asshole - who laughs after their kid gets slammed like that?

Mother/Father of the YEAR !!!

Well, one reason for the laugh could be that if you laugh some when they fall, they won't burst out crying for what could be nothing and then in the future won't just jump straight to crying every time the smallest fall happens. They learn to laugh it off and just get back to having fun if they aren't actually hurt.
Then you can still go over and give em a hug and make sure they didn't actually get hurt. Most times it's just a little bruise (maybe) with kids. Kids are tougher than people realize sometimes.