Then do it. Call 911, say there's an injured, unattended child at the playground, and you're getting a hostile response from folks as you try to locate the guardian so you'd appreciate it if a social worker collected the kid until the parents can be found.
There is nothing illegal about speaking to a child, and when you soft play people like this you empower them. Let them have to show a cop a DL to get their kid out a squad car to learn their lesson if they can't handle polite help.
(Also, what is this narrative around HN about being accused of nefariousness at playgrounds? I used to eat my lunch at one near me because it was the only park with a trash can nearby and I didn't want to lug my trash back to my apartment before going on my way towards the city -- nobody ever said a word to me aside from asking for a ball if it rolled over.)