|I think the main difference is that in the 'winning' image there is a little better composition, the girl is sharper and a stronger presence, and the background is more blurred. Your image is excellent, however the children need to be sharper in order to 'pop' the image - perhaps trying one child rather than two would be easier in this case - and the background draws too much attention to the eye, so we become dizzy by subconsciously trying to focus on the playground equipment. In an image with such movement and eye trickery, its important to give it a really clear and possibly larger focal point. Personally I don't like the overblown sky coming through on either image, but as its basic editing there's not much you can do about that.
I think your idea was excellent, and you are incredibly dedicated to get up with your expensive gear onto a roundabout and still be able to take a great photo!
Sadly we have no roundabouts in our town.....bummer......I've been dying to try it myself.
Don't worry too much about the voting though - it is all ridiculously subjective and there are a million professional quality outstanding images on this site that have never received a ribbon. Frustrating, isn't it.