From the National Parks website:
ABOUT EVERY KID IN A PARK
Did you know that you own millions of acres of national parks, historic structures, cultural artifacts, ancient forests, snow-capped mountains, and clear blue lakes? Our federal public lands and waters belong to all Americans and are waiting for you to explore them! You can get your Every Kid in a Park pass at www.everykidinapark.gov, the official website where parents, educators, and kids can learn more.
To help engage and create our next generation of park visitors, supporters and advocates, Federal Land Management agencies, launched the Every Kid in a Park initiative. The immediate goal is to provide an opportunity for each and every 4th grade student across the country to experience their federal public lands and waters in person throughout the school year.
Every year, beginning September 1, all kids in the fourth grade have access to their own Every Kid in a Park pass at www.everykidinapark.gov. This pass provides free access to national parks across the country.
The Every Kid in a Park pass is good for the 4th grade school year, until August 31, 2018. Information on obtaining the pass is available by visiting www.everykidinapark.gov.