http://www.riderplanet-usa.com/atv/t...3/map_41a9.htm
The above is to a rider map that shows the campgrounds. I used to go to ICE Water Campground. There is a kids riding area inside the campground and the trail head is right across the entrance from the campground.
You can get maps at the ranger station in Cle Elum.
EDIT: what is not intuitive, is you take the Elk Heights exit from I-90 and go east on the NORTH side of the freeway on Thorp Prairie Road. At the bottom of the hill, you cross back over the SOUTH side of the freeway go up West Taneum Rd until you reach the campground on your left.