EXCLUSIVE: Reese Witherspoon & Matthew McConaughey Share Parenting Advice, Dish on Their Grumpiest Kids

The 'Sing' co-stars spoke with ET on Monday where they opened up about their talented (and sometimes adorably temperamental) children.

When it comes to parenting, Reese Witherspoon says tough love is the best love.

Witherspoon and her Sing co-star Matthew McConaughey sat down with ET's Cameron Mathison during a junket promoting their upcoming animated musical on Monday, where the A-listers opened up about their child-rearing styles.

"We have rules around the house. I always say if you aren't yelling at your kids you're not spending enough time with them," Witherspoon joked. "[I'm] just telling everyone to clean up all the time."

"People know not to call me before 8:15, because 8:15 is when everybody needs to either be on the bus or at school and then my whole life starts at 8:15," the mother of three added.

WATCH: Reese Witherspoon Brings Lookalike Kids to 'Sing' Premiere, Talks Singing Katy Perry & Taylor Swift

In reality, however, it sounds like the Oscar winner's youngest son, 4-year-old Tennessee, is actually the temperamental one in the house.

"You're not allowed to talk to him before 8 a.m.," Witherspoon explained. "You can't sit close to him either, he gets really mad. He needs like, an hour of alone time. He drags his toys and blankets into the room and sits there by himself."

McConaughey said he can relate. According to the star, who also has three kids, his youngest son, 3-year-old Livingston, has to "wake up very different" from his 6-year-old sister, Vida, and 8-year-old brother, Levi.

"It can be a long process to get young Livingston to come and greet the day without going into a rage," said McConaughey.

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While he might have a fiery side in the morning, Livingston is also the most musically inclined among the star's kids.

"He has the ear," McConaughey shared. "He sat down at the piano and from day one could go tell a minor note from a major. Any music you play, any chords you get going, he can find a melody that falls right in track with it."

For Witherspoon, it's her 17-year-old daughter, Ava, who has the musical talent in her family.

"She plays a lot of different instruments and has been to lots of different music training camps. She can write a song, she can play any song on the guitar or piano or ukulele," Witherspoon marveled.

WATCH: Reese Witherspoon Shares Cute Photo of Son Tennessee on Set: 'Introducing My New Makeup Man'

Music, of course, played a big role in the stars' upcoming film, Sing, in which Witherspoon stars as Rosita -- a mother pig who gave up on her dreams of becoming a singer to start a family, only to find an unexpected opportunity in the form of a singing competition organized by Buster Moon, a koala voiced by McConaughey.

For her role, Witherspoon had to practice and practice to get her singing voice up to snuff, and she said he kids "were very proud" that she pulled it off.

"They had to endure me singing 'Firework' around the house 400 million times. So they were happy to see the end result," she added.

PHOTOS: Stars Share Pics of Their Cute Kids

Witherspoon's hopeful porcine character struck a particular chord with Livingston. McConaughey revealed that Rosita was actually his youngest son's favorite character. Meanwhile, Vida's favorite was Ash -- a porcupine played by Scarlett Johansson -- while Levi was a big fan of a snail character who sang "Ride Like The Wind."

"I got a second place on some of [my kids'] lists," McConaughey quipped.

Over the weekend, ET caught up with McConaughey at the premiere of Sing in Los Angeles, where he opened up about his family's hectic holiday plans. Check out the video below to hear more.

hits theaters Dec. 21.