Had this happen to me when my kids were around 6, in Utah!

I saw a dude in park and he was on his phone, acting like texting but holding his phone up vertically. Nobody texts like that. My girls were in bathing suits in a splash thing.
I took a little stroll over to the bushes behind him where he was sitting.
I came up on him silently and SAW HIS CAMERA was pointing at the kids. I let him know through elaborate words that I had been filming HIM too. He took off quickly but only to the other side of park!!

I talked to a few moms and said THAT GUY THERE is taking photos of kids. They said OH NO, that guy is always here ...
(this was a one-time visit for us).

I think this is way more common than most know.... people in parks, if they are not with kids, you need to casually walk behind them and see if you can look at their phones!!