Summertime in the DC area means hot, humid weather, but kids love to play outside.
The perfect solution is a sprayground: a playground with water features designed for kids to play. There are a few in the area (3 in Arlington), but Hayes Park is our favorite for the shade and the presence of bathrooms.
From the county website:
"Park featuring a summertime accessible Spray Park. The Spray Park is open from Memorial Day to Labor Day from 10:00am to 8:00pm. Please note that hours of operation for the Spray Park may vary on holidays. 2.86 acres."
In addition, there are restrooms (clean and serviceable, but not amazing), water fountains (I'd still bring drinks from home), a picnic shelter, picnic table, and school-age playground (combination school-age and pre-school, enclosed).
I found Hayes to be a pleasantly small, contained park that is easy to navigate. The sprayground is tucked to one side with lots of shade and nice benches for those who want to sit out. The theme of this area is the solar system, and the water cycles through different phases, which keeps kids guessing and moving. While there, we saw kids from infants to at least 10 (and maybe some grown-ups) enjoying the water.
The dry area of the playground was a good size for pre-school and school aged kids, and was clean and well-kept. This area was less shady, but not brutally sunny. It's summer: bring sunblock and hydration.
Free on-street parking.