Anyone with a permanent disability can now receive a free, lifetime pass to all of the National Parks and Federal Recreational Lands. The disabilities listed on the website include physical, mental, and sensory impairments. There is a $10 processing fee.
There are over 2000 Federal sites that can be visited throughout the US the nation.
If your child has a disability and you get the pass, it would qualify everyone in the car for free admission.
There is an application to complete, you can find it here.
The website offers a great deal of information, including discounts for some additional services.