"Gideon and Harper turn 6 today!" Harris wrote on Wednesday alongside snaps of the two going down a water slide. "Happy birthday to these big, bold, brave, bright, beautiful bundles of bountiful blessings. Boom. #balliteration @dbelicious and I love you kids SO much! #grateful."
Burtka took to Instagram a day later to also gush about their twins.
"Happy Birthday to the most loving, fearless and most adventurous eaters
I know!!" Burtka wrote. "@nph And I love you Gideon and Harper!"
Harris and 41-year-old Burtka married in Italy
in September 2014. They welcomed their twins in October 2010 via a surrogate mother.
"So many feels: excited, nervous, giddy, wistful," Harris, 43, tweeted about the big moment. "But mostly proud. Can't wait to see who they become."
See more adorable celeb kid back-to-school pics in the video below!