It sounds like maybe your park is more spread out than the ones I go to. My most frequented park has one climbing structure and it is suitable for all of the kids as long as they are steady on their feet and ready for climbing (usually around 16-18 months). I bring a ton of sand toys (lots of multiples of the same thing) and rarely have issues with fighting even when other kids come to share...at least nothing that can't be worked out fairly easily. I go with 7 kids when my own two are there and there are four swings, so we take turns. From the swings, as long as I stand behind them and push from there, I can see the others playing and if I need to dash over to the structure to help someone, the others are safely containedin the swings. For a park that is more spread out, I would keep them all together and do as Playfelt suggested...move from one area to another all together. I do have one park that has teeter totters which are quite far from the structures, so we all go there first and take turns on them and then we head all together to the play structure area.