I think it's up to the parents and the children. When my nephew was 2 I 'helicoptered' him around my local park. If I hadn't there was some equipment he wouldn't have been able to use as he wasn't very good at climbing steps etc. He also would have pushed other children if they were in his way. I saw it as my job to be there to stop pushing before it occurred, or if that was not possible to apologise to the child/parent, and take my nephew off the equipment if necessary. I also think he had a lot more fun with me there smiling, waving, explaining etc than he would have done if I'd left him completely alone.
Aged 3 he still pushed so I had to be nearby to teach him not to, and to teach him how to take turns. Aged 4 I think he would probably be alright but while I might not stand right by him I would still make sure I was constantly watching him so I would know exactly what had happened if he did misbehave.
Another nephew of mine I took to the park when he was six. I didn't 'helicopter' him but he wanted me to join in his imaginary game of sailors, so I did. We played for about an hour, and I don't think he would have had as much fun if he had played by himself.
When my niece was 6 my mum and I took her to a park she didn't know. It was quite spread out and busy, and when my mum and I sat down my niece would go and do one thing, such as one turn on the slide, before coming back to us. The equipment she hadn't used before she wouldn't go on. So I went with her and she was suddenly a different child - using everything and talking and laughing. She was just a bit shy to explore properly on her own.
I personally think that if you're happy being with your child, and the child's happy to have you there then everyone has fun. It's only a problem when the parents being there means the child can't play properly because the parents are always saying "no, that climbing frame is too big for you," or "don't climb the steps, Mummy will lift you," etc.