In the animated movie, Witherspoon lends her voice to a harried pig named Rosita with big singing dreams. But Witherspoon looked anything but stressed on the red carpet, sporting a black dress embellished with pink, gold and white stars, with her mini-me daughter, 17-year-old Ava, in a strappy black dress by her side.
The 40-year-old actress' sons, Deacon, 13, and Tennessee, 4, also accompanied her, looking super adorable in their complementary outfits, pairing dress shirts with checkered Vans sneakers, while her husband, agent Jim Toth, wore a dapper sweater and suit jacket combo.
ET's Carly Steel caught up with the Draper James founder at the premiere, where she revealed her favorite karaoke song (Sia's "Chandelier") and that even if her kids are embarrassed by her singing Drake songs, they do give her cool mom credit for one thing.
"Well at least I know how to work Snapchat," Witherspoon tells ET. "I think, you know, most moms don't. [My kids] are like, 'You actually know what Snapchat is and you're actually sending us a lot of Snapchats.' So, maybe that's kind of a cool thing."
Speaking of motherhood, Witherspoon's character in Sing is also a mother -- of 25! -- which gave her a way in to the role.
"I think that there's something beautiful about the idea that your mom is a person too," she says. “Beyond being just a mom, [Rosita] has her own dreams and desires so… the film kind of explores that through my character."
Playing Rosita also required the music-loving actress to belt out some big tunes, covering songs by Katy Perry and Taylor Swift, but she says she gave the two hit-makers the heads up when she tackled their tunes.
"I actually ran into both of them as I was doing each song," Witherspoon revealed, adding that she told Swift, who is a friend, that recording "Shake It Off" gave her a new appreciation for how hard the song is to sing. "[Swift] was like, 'Yeah, you know, I make it look easy but it's really hard,'" she said.
Witherspoon also spoke to Perry about her hit, "Firework." "She's like, 'That's hard.' But I just did the best I could. I'm supposed to be the mom singing the songs, so they don't have anything to worry about. I'm not cutting an album any time soon!"
With the holidays around the corner, the mom of three reveals her plans involve family time, gingerbread houses and more singing! "Maybe [we'll] have a little party," she says. "Sing carols. I like to sing carols."
Last month, ET caught up with Ryan Phillippe, Reese's ex-husband and father to Ava and Deacon, about how they share co-parenting responsibilities through the holidays. Find out what he had to say in the video below.