In short: I spent the afternoon here with a 10-year-old, 7-year-old, 5-year-old, and 1-year-old, and every single one of them had plenty to do... plus, they begged to come back the next Sunday. A++.
(If you're using GPS to get here, I advise aiming it at the dog park -- 251 Bickerstaff Way, Gaithersburg MD. When I tried the playground itself, my GPS landed me on a random spot on the highway adjacent to the park. From the dog park, continue down the road; the playground is on the left, and there is street-side parking right next to it.)