Little practice and a decent hammer throw technique your average adult ought to be able to get higher than a six foot arc with 50lbs. Trick is to grab em by the ankles, looking them in the eye is incredibly distracting.
I can lift a 50 pound weight over my head, multiple times in a row. I'm not sure how these parents are actually doing it, I assume the kids climb over, and the parents aren't actually yeeting them.