Powered Outdoor Speakers for Kid's Trampoline

Wanting a means of plugging an iphone/ipod (thru any means, including 1/8" mini) to some speakers along the trampoline. You know - to entertain the kids and bug the neighbors (I might enjoy it too). Any suggestions? I'm coming up with nothing in the cheap (~$100) range that I'm seeking.

Power source is 100' away, which means an extension cord most likely. If powered speakers, they must be weatherproof. Another option might be a boombox with aux input, that could be moved to and from the trampoline while playing.

Thanks for any ideas. Peter