The 36-year-old singer also opened up about son Titan's incredible talents, and why Serena Williams' baby is going to 'run the world.'
Beyonce is taking her post-baby body seriously!
ET's Deidre Behar caught up with Kelly Rowland at the premiere of True and the Rainbow Kingdom in Los Angeles on Thursday, where she opened up about hitting the gym with Bey and JAY-Z after they welcomed their twins, Rumi and Sir Carter.
WATCH: Beyonce Hits the Gym With JAY-Z and Kelly Rowland -- See the Pic!
"Everybody goes hard!" Rowland said of their workouts. "It's a whole bunch of us who go and work out, and we have a really good time."
"We got to Angela Davis' SoulCycle. She's incredible. She pushes you. SoulCycle is just this really incredible, mental workout that is just awesome," she added. "But Angela Davis, she is the master. There's no one like her."
As for Bey and Jay's twins, Rowland said they're just as adorable as we imagine. "You couldn't expect anything less," she gushed.
The 36-year-old singer brought her own son, Titan, along to Thursday's premiere for a "mommy date."
EXCLUSIVE: Kelly Rowland Dishes on Parenting Advice She Got From Beyonce & Other Famous Moms
"I love being with my son. He's excited. I told him we were coming to see a movie. He loves the movies. He surprisingly sits down and is quiet in movies, which completely blows my mind," she said of her 2-year-old, adding that he's a "really interesting cross" between her and husband Tim Witherspoon's personalities. "He's both of us."
Rowland, who is "so excited" to be back in the studio recording new music and is "inspired by life," said Titan is taking after her in one area: "[He's] very musical."
Titan definitely has a good ear -- Rowland shared that he's already bilingual, and she's working on language No. 3!
"He is fluent in Spanish. I'd like for him to start French. My nephew actually speaks French. And we're looking forward to, hopefully, a Diego birthday party. A Diego-themed birthday party. We'll see what happens," she said of his upcoming third birthday in November, before gushing about her friend, Serena Williams and her soon-to-be baby.
"Hands down. She is going to be an amazing mother. I am so excited for her and motherhood," Rowland confessed. "It's natural for her and, this baby is going to probably run the world."
"It's more so the mental of all it, you know what I mean? It takes a hell of a woman to be with-child, and play and winning the Open. So, incredible," she marveled. "Incredible."
See pics from Williams' recent baby shower in the video below.
True and the Rainbow Kingdom is available exclusively on Netflix on Friday.