After some Google searches, I learned what I saw last night was that month's Lullaby Moon performance by Lucia Neare's Theatrical Wonders. Sponsors included the Mayor's Office of Arts and Cultural Affairs and Seattle Parks and Recreation.
They're offering a few more performances this year, on August 20, September 18-19, and October 18. They're all free, and all outdoor.
It genuinely was wonderful, and would be ideal for anyone with children.