This picture of a playground roundabout demonstrates just how popular they are with children - very well used!
PlayPumps International have come up with the ingenious idea of harnessing the energy of children playing on a roundabout to pump water from deep underground (40 to 100 metres) to a water tank 7 metres above ground. People can draw water from a tap instead of walking for long distances to a source which may not be clean, and any excess is diverted back down the borehole.
The system is paid for by in part advertisements on the two sides of the water storage tank. The other two sides carry public service related messages. The advertisements are fairly strictly controlled, which would be one of my concerns.
Otherwise it seems to me a wonderfully ingenious way to utilise the energy of children. If it had been available years ago, my first son could have provided heated water all by himself!