We were recently on a trip to Death Valley National Park (yeah, I know, we like to pick the weird ones) and were pleased to discover the Access Pass.http://www.nps.gov/fees_passes.htm
This pass is free and allows families with a disabled person to enter all national parks free of charge (with half price on any camp sites). The website says you need some kind of documentation, but a woman in the visitor's center saw Ewan's mobility scooter and asked if we had the Access Pass. When we asked what that was, she refunded our park entrance fee and issued us a pass without any further documentation.
We plan on hitting a lot more of the local national parks now. Something to think about for anyone looking to plan a family vacation this year.