All up in my space!

I am at a playground at a church where the little girl I babysit takes ballet. While waiting for class to start, she and her little brother play on the playground. Of course there are other kids here. The other kids keep crawling under the bench (metal frame, cloth seat) where I am sitting. They will put their knees and hands up and push on my butt or kicking my legs. And they are pushing over the kids’ backpacks I have sitting by me. When they come out from under the bench they crawl between my legs! I have my feet crossed, but they force their way through. It took me by surprise, so i wasn’t really sure what to say at first. I find this really annoying and uncomfortable, especially since their parents are watching and not saying anything. These kids are like 3-8 years old. There is not another place for me to move to sit where i won’t be in the traffic area except a small space on the floor. And I don’t want to say anything to the kids, even politely because I know how some parents can be!