Greenwood Park in Des Moines has a great shaded playground as well as a splash pad, which operates from May to September, 9 am to 8 pm. The splash pad is fun but slippery, so I recommend wearing water shoes. There's a parking lot and seasonal restrooms near the splash pad.
The park is quite big and the map below shows where the play area is. It's very close to Bergman Academy, so I try to avoid the play area during the school year because it's often crowded with students.