In the park today - two swings, about 30 kids. Two girls that I know by name and are aged 5 and 7 were on the swings. Their mum was chatting on the other side of the park and not someone I know other than to say hi to if we walk past each other. My boys aged 3 and 6 were waiting for a turn. After about five mins of them waiting I went to stand with them. After five more mins I said to the girls "could you give someone else a turn now, x and x (they know my kids names - same school) have been waiting a while. The cheeky older girl said no, it is important to be patient. 
. I was a bit taken aback but said yes that's true, but if you show kindness then it means next time you are more likely to have it shown back to you when you are waiting for a turn. Anyway after a few more mins and me asking again they got off the swings. My boys had three or four mins on the swing then another kid was waiting so I told them they had to get off - we were also heading home anyway.
What is the etiquette though? It was the kind of situation where if I had gone over to the other side of the playground to ask their mum to make them give other kids a turn it might have caused a scene. No reason to think it would, but it might have, and I would hate to ever have weirdnesses going on with me and other regulars in the park.
Would you be okay if another parent asked your kids for a go? I think I would, providing they had had a proper turn.