One of our favorite family hikes is Silver Falls trailhead in the Okanogan/Wenatchee National Forest, 30 miles up Entiat River road.  Silver Falls is a popular hike with locals and tourists who want an easy to moderate hike the whole family can enjoy. The trail may be an uphill climb, but there are plenty of spots to cool off, rest and admire the cascading creek barreling down the hillside.  Younger children may moan and groan a bit for the effort they have to put forth to walk the entire trail.  But if you make it to the main attraction, where the falls are actually rolling over the rock face, you can then take them back down.  

If you make it to the top of trail above the falls (3,100 feet elevation), a pleasant canopy of trees will cool you off and you can splash your face with cold water.   Your hike down the other side will have a different vantage point with switch backs that lead you away from creek and back again. Total 1.8 miles.  Definately worth the trip for the views and the excercise.