The dinosaur park in PG County, Maryland is a very small fenced area where few years ago a builder discovered remnants of an Astrodon. Unlike other areas in the world or the United States where general public cannot easily access such sites, this particular one is open to public on the first and third Saturday of each month from 12pm to 4pm and you can actually meet real paleontologists and researchers. For elementary school kids or other dinosaur aficionados, this can be a worthwhile place to go to on a spring afternoon. There is no entry free and you can even get to keep some of your findings!
Though there is no playgrounds for the kids or any of the regular fun stuff you find in a Smithsonian museum or an amusement park (like Dinosaur Land, Virginia), kids can have a lot of fun in the dirt, going through stones, fossils and learning about how fossils get their impressions, identify differences between ironstone, lignite with plant impressions, bone fossils etc. It is a very good learning experience.
You can even surprise yourself by finding a real fossil as well. For example, the day we went there, we found an Astrodon tooth in the first half hour we were there and the volunteers there thought that was very impressive. Few other visitors that day found pieces of an Astrodon claw as well. If you find a real dinosaur fossil in the park, you will need to register it with one of the volunteers - it will be sent to the Smithsonian for evaluation. If it does end up in a museum in DC, I am sure your name will get there as well.