I also seem to recall a separate Yerushalmi (maybe in Moed Kattan) that described a game that kids would play where they would bounce a ball off a wall and it would hit and kill each other. I thought that that was what this Yerushalmi was referring to at first.
This was back in 9th grade when I went through the Yerushalmis in Moed (except for Eruvin, obviously). I haven’t been able to find that Yerushalmi again since, so take a 10-year-or-so memory for what it’s worth.