https://en.wikipedia.org/wiki/Meitiv_incidents

Tldr: Officials clarified that it's fine and they shouldn't have been bothered beyond police asking the kids if they were okay in response to a call.

The police and CPS responded because someone called the cops, who are required to respond in some way and then to document the case. The reporting code for "report of unsupervised child" is intended to be "neighbors haven't seen the parents in several days, but they noticed the kid moving around the house and were concerned". Sometimes it's not okay for kids to be alone.
So the police responded because someone called, and then gave them a ride home and filled their report. CPS got the report because the only category it fit in was one they are supposed to investigate. They did their investigation because the law says if you're under eight you must be supervised by someone at least 13, and because they were in violation they had to do their follow-ups, which are invasive because they're geared towards actual issues and there's no way to delicately inspect someone's home and interview their children.
When it happened again at the park, there was now a report on file for a CPS investigation that was still in progress, so now it's "parents being investigated for neglect getting another report of the same behavior", which means that now the presumption is that the parents aren't capable of following a directive to not do the behavior that started the investigation , so instead of sending them home and then sending an officer to see what's up they're going to hold them until they can determine safety. Which they were, but all the people see is "they were instructed and agreed to not leave them unsupervised until we finished and we got a concerned report about them being left unsupervised".
Eventually officials clarified that CPS was incorrect, and that the laws wording and intent was to prevent young children from being unsupervised in vehicles and structures, not parks, sidewalks or in public. No leaving your 7 year old home alone or in the car.

First incident is on the busybody who called the cops and the CPS people who didn't just leave and drop it when they learned they weren't left behind at home or in a car, and that the sidewalk and park weren't like, a highway median and an industrial park.

Second incident is a little more on them. Preposterous or not, they were explicitly and legally informed they needed to not do that until CPS got back to them, and they agreed to do so. It was still more of an ordeal than it should have been, but you should generally not be surprised when they respond poorly to you doing what they just told you not to do.
You can be entirely in the right and end up in more trouble for not following instructions during the process of figuring that out.