An old guy starts playing with your kid at the park

There have to be more information than this for me to decide what to do. I might try to ID him to find out more.

And depending on what I find out from his behaviors, I might do nothing, might call the police or might scare him myself. It depends on my conclusions.

I have seen plenty of harmless people who can't have kids behave like that, also neurodivergent, and few harmful ones. It is generally indicative though of poor social familiarity and judgement.

I too sometimes play in the park but I have kids with me I am watching over. I don't do it with strangers. And not in a foreign country.

I'm the oldish guy (under 50) with kids still under 10. I generally don't go to the park without my kids. I do have random kids around when I ride my BMX whether mine are around or not. Very Small town, generally first question I get asked "you're so and so's dad?". Generally answer is yes, if parents are around I deliberately go interact with them.

I would judge how he interacts with them. If it's in a genuine "grandfather way," then I would think he just enjoys being around children that remind him of his own grandchildren.
If he's overly friendly, then it's probably best to leave.

As a child I was brought up never to talk with strangers that also applied to interacting with them, I would be more than suspicious and discourage making friends with strangers in parks.

Nope..

it depends maybe he just enjoys watching / helping kids have fun it could be that simple

If the "kids" are big enough to activate the teeter-totter leverage with the old guy, I think we're ok.

Yeah I wouldn't make it past sand on the slide.

I’d worry simply because he should know better. It’s not a good/safe look.

I would tell them that it’s time to go home!

Who puts sand on a slide ffs??