Dewitt and her husband, Office Space star Ron Livingston, adopted their eldest child, 3-year-old Gracie James, in 2013. The former Mad Men star says her two girls are getting along just fine.
"They're happy," she notes. "They like each other a lot."
DeWitt and 49-year-old Livingston met while filming the FOX television series Standoff in 2006. They've been married since 2009.
DeWitt also talks about her new film, La La Land, with DeGeneres, and her experience working with Ryan Gosling.
"He's so unattractive ... he's hard to look at," she cracks. "Ryan's super fun to work with but he's perpetuating a story that's just not true."
DeWitt explains that Gosling has been going around saying she put acorns up his nose in one scene, though she calls the story "just not true."
"I asked him in the scene -- 'Here, put this up your nose,' and he put them up his nose -- but I wouldn't put acorns up Ryan Gosling's nose," she says. "I suggested it, and he did it, and then I was like, 'OK, game on, this guy's gonna be fun to work with."
Aside from Gosling and DeWitt, La La Land's star-studded cast includes Emma Stone, J.K. Simmons and John Legend. ET spoke to Legend earlier this month about the critically acclaimed film, and he said that he and Gosling bonded over fatherhood on the set.