Try http://www.trebuchet.com

However, the design you're looking for won't be easy to build.

Let me put it this way... My trebuchet took a year to build, weighs around a thousand pounds (with no counterweight), is ten meters tall, is presently equipped with an 800 pound counterweight and probably cost me two grand to build, with big-time discounts on the wood.

My trebuchet will throw a 10 kilogram weight about a hundred meters.

So the project you want to build is a fairly large undertaking.

My first trebuchet was about three meters tall and had maybe 50 kilograms of counterweight on it. I could throw half a brick about the distance you want (50m). That would be a much better 'family' project.

If you e-mail me, I can give you some general guidelines that might be helpful. I didn't listen to a lot of the advice people tried to give me on trebuchets when I first started building my big one, and as a result it's a piece of crap... hehe

Adam