When I've played with my nieces and nephews at the local park, not only did I not get any negative comments, I had a lot of moms say they thought it was great that I was going all over the playground equipment down the slides, etc. along with the kids, and that they wished they had the energy to do that too.

Not saying this kind of thing never happens - there are jerks everywhere. But I'm a hairy bearded man in my early 30s with a pretty bad RBF, and I've never had anything like that happen after many occasions. Your attitude, body language, and general demeaner matter a lot. I genuinely enjoy acting like an idiot while playing with my nieces and nephews and apparently it shows.