NSFW I need advice from a cop or anyone in law enforcement about how to handle this situation. Rest is graphic.

I was walking my dog along yesterday when I heard coughing several times and did not pay any mind till it seemed intentional. I looked by the childrens playground and noticed a man behind the fence masterbating and looking my way. Of course I quickly grabbed my phone and I was getting ready to call the cops when I wondered if this would be a 911 stituation. There were no children there and he already ran off to the woods when he saw me grab my phone.
He could’ve mistaken me for a kid ( I’m only 5 ft and from a distance I have been mistaken for a kid more than I can count)
Either way he is a perve. I didn’t have good visual because as soon as I noticed what he was doing, I looked away in discust.
Anyhow, back to the phone. I didn’t have the local cops number on my cell so I wasn’t sure who to call. I then ran to the main office and reported it. They said they would let the patrol know so they can beef up security.
Anyway, I am also worried. There are a number of children who go off to play alone at that park. What if this person is a child pervert.
Should I let the office handle it or should I inform the parents of the children I have seen roaming out alone?
The office did not release this information to the people living here and I doubt they will because parents will probably want to move.
I not sure what to do. I don’t want to get in trouble with the office once parents start to inquire and I don’t want a child getting possibly hurt.
And if ever I see something like this, do I call 911 or the local cops? He may not even come back. He probably thought I was calling the cops and that is why he ran off.
How can I resolve this and not get in trouble for it?
Thanks for any advice.