The actress, who delivered her second child with art advisor husband Will Kopelman on April 22, says her recovery with newborn daughter Frankie was quicker this time around than following her first pregnancy with elder daughter Olive, 20 months.
"I knew that I have two kids to take care of now and just kicked my own butt to get back into the swing of things immediately," she tells PEOPLE in this week's cover story.
The biggest challenge? "Just being split. They're awake and asleep at different times, or awake at the same time, and it's a juggling act a little bit."
Barrymore, 39, says her own tumultuous childhood influenced what she sees as most important for her kids.
She wanted Olive "to have a sibling, because I really would have liked that in my life," she says. "And always being on time, being there when I say I'm going to be there. Creating a real stable home. I know when my daughter wants to swing higher and jump into the pool and all of these things, she feels safe. To make your kids feel safe is everything."
Barrymore says Olive is relishing having a baby sister – so far. "Olive loves Frankie and just wants to hang out with her all the time," she adds. "She wants to feed her. She's super into it. That's now. I'm sure things will ebb and flow."
Now promoting Blended, her new comedy with Adam Sandler which hits theaters Friday, Barrymore admits she wasn't sure she wanted to act anymore after Olive's birth.
"I didn't think I could go back to work, honestly. I didn't want to be away from my kids and it just wasn't the priority," she says. "With Adam, he makes everything really safe because he really creates an environment where kids can come and be a part of it. It was a really big lesson for me."
Knowing duty would be calling so soon after she gave birth "kept me at least physically present and sharp," Barrymore adds. "As opposed to being a brain scholar, which I am not right now. As I walk into a room to get something and pass the doorway, I've forgotten already."
For Drew's album of intimate family photos with Frankie, Olive and Will – and more of her exclusive interview – pick up this week's issue of PEOPLE, on newsstands Friday