The best way to reserve an outdoor birthday party venue with swings is to register with one of DC's parks services. Spaces like Lafayette-Pointer Recreation Center have multipurpose rooms, playgrounds with swings, and access to parking, tennis courts, and picnic tables. But you'll have to reserve your spot ahead of time if you don't want to share with the general public.
Some Washington, D.C. parks are designed with access for everyone in mind. Clemyjontri Park is disabled-friendly, with wheelchair and walker access throughout it. Besides swings with safety features installed it also offers a maze, monkey bars, and much more!
For a single venue that offers something for everyone, check out Wheaton Regional Park. Renovated in 2020, the park boasts facilities for several sports activities, including tennis, hockey,
football fields, ice skating, and baseball. Kids can play on the swings, explore the Brookside Nature Center, and ride the Ovid Hazen Wells Carousel for their birthday.