Although I appreciate more "natural" garden fixtures, I also accept that kids want colorful, fun stuff to play with. I found a link with some reasonably priced extras if you are building a playground ( mountable steering wheels, periscope, swings, slides ). They also have something like 12 colors of recycled rubber mulch.

Here is a link that might be useful: Playground goodies

I'm sure you don't plan on using the tire mulch around plants, but just in case anyone reading this is thinking about it, DON'T DO IT. The rubber retains heat like crazy!

-B