This has been a nodeshell commando action
They play on the monkey bars.
They dance in the tire swings.
They woosh down the slides.
They leap merrily across the gravel.
They break people.
Playground elves are the source of many a playground accident, incident, mishap, or other such unpleasant occurance involving monkey bars, slides, or those really unstable ring things that people are supposed to swing on, or play with (?).
You can't see them, but they're there. They're the reason that the slide will lose a supporting beam, and curve when it's not supposed to, flinging you out to the gravelly earth, only to realize that, although faulty at the time of the incident, nothing is wrong with the slide at all.
Playground elves are responsible for swing accidents. When a tire swing breaks and crushes someone's legs, that's them. When someone jumps out of a swing and their leg catches on the seat so that they're sent flailing through the ether to land crookedly and break their arm, the playground elves are there.
Most importantly though, and the reason that all playgrounds are plastic now, are the big, metal jungle gyms. These monstrosities used to grace many a playground, but usually, a few months, or a year after they were built, the elves would get wind of it. Next thing you know, some kid has broken his neck on a jungle gym. The bars that are missing from your local jungle gym? That's all them. They remove them tactically, so that the kid who's climbing, and not exactly paying attention, may just fall through, a tiny body slamming into the ground... that's the greatest pleasure of the playground elves.
Countermeasures, such as the influx of recent plastic playgrounds has given the elves much less to work with, but ingenuity remains, and at some point or another, they will find new, creative, and sadistic things to do with this new technology. In the future, it'll come down to us or them. We have to stop the elves here. Now. Or we'll have to get rid of playgrounds for good.
Hence... war is once again, inevitable.