Yah, we play with everyone. But there ARE some reasons why I would not help a kiddo with their wants.
First, I am not always on the playground (proper) with my kids. I'll sit on the grass or bench to the side, watch 'em intently, and give my kids some space to explore by themselves. Sometimes I'll get in there and climb around, play tag/chase/monster/princess/dragon/cheetah/etc., but sometimes going to the park is my of the opportunity to rest my body while the kids run off their (never ending) energy. Legalistically, this makes it difficult for me to help another child to swing, or what have you.
I'll be honest here, my kids have to TWIST my arm to get me to push them on the swing. Okay, granted, when they were still little I'd help them of course, but now that they are 4 and 5 they pump their legs and/or I'll give 'em a few underdogs and then let them do their own thing. It's really strange, but pushing people on a swing is one of my least favorite games to play with children.
But IF I'm playing with my kids on the playground (or grass/trees around the playground) I certainly invite other kids to play with us. The more the merrier! It's a great exercise for the kids in how to be socially inclusive, flexible, to play at the level of younger children, how to respect other people's boundaries, and how to make mayhem and fun! Same at the beach - if we're in the water looking for creatures, we make sure to let other kids have a touch/see too. I like to keep a good look out for any little ones around me. It only takes one moment with their parent's eyes off of them before they fall into the water, etc. I sure do appreciate when we take a village attitude towards our children's immediate safety.
I like it when my kids interact with new adults too. I'm always around to help/hinder certain exchanges, and I try to emphasize to my children that they get to listen to their gut instincts. They are both incredibly social and friendly little people, but if they feel a funny feeling around someone, that means something is wrong and that they need to be distant.
When I see another parent hanging with my kids, I wave and make sure they know who I am and who I belong to. I figure that if the parents had A. issue with my kids, they'd send body language or actual words that would express disinterest/distaste for my kids. It's important to me that my kids learn that we have to respect other people's boundaries and not force ourselves onto people.
Anyway, the short answer is that yes, I make a point to help and have fun with any kids we happen to meet on the playground.