The 47-year-old singer shared posts Sunday on Instagram Stories from the epic bash she threw Zuma ahead of his ninth birthday Aug. 21.
Videos show Stefani's house decorated with Hogwarts house banners, life-size cutouts of characters from the films, a replica of Platform 9 3/4, and plenty of treats, including Zuma's Sorting Hat birthday cake.
Stefani shares Zuma, 11-year-old son Kingston and 3-year-old son Apollo with her ex-husband, Bush frontman Gavin Rossdale. She opened up about her approach to parenting in an interview with People in July.
"I was in a band and free my whole life, so I always thought I would be really free with [my kids] and do whatever I wanted, like, 'Oh, I'll take them out of school,'" Stefani told the magazine. "But you learn that when they have boundaries is when they feel the safest."
"I see a constant fear of 'How am I gonna make sure these kids are good people?'" she shared. "I love when they still play and you're like, 'Oh good. They're still doing that.'"
Stefani split from Rossdale in 2015, and has since started dating country star Blake Shelton. Shelton took the singer and her sons on a private helicopter ride in July after visiting the Chicksaw Cultural Center in Sulphur, Okla.