The sisters talked the relationship between their two kids — Hilary’s son Luca Cruz, 5½, whom she shares with ex-husband Mike Comrie, and Haylie’s 2½-year-old daughter Ryan Ava Erhard, whom she shares with fiancé Matt Rosenberg — at the Words With Friends 2 launch party in West Hollywood, California, on Thursday.
“They really play well together,” Haylie, 32, told PEOPLE. “One time I had Luca for a sleepover and the two of them were outside running around on a Slip ‘n Slide, and I remember looking out at them in the yard being like, ‘This is what they’re going to do as teenagers.’ ”
Adds Hilary, 30, “Haylie sent me a picture and Ryan had no shirt on, a diaper and a flower crown. I don’t know what [Luca] had on, but they looked like they had been playing so hard. She was like, ‘Yo, Coachella 2030.’ I’m like, ‘No! Stop!’ If this is a flash into our future, we should be very afraid.”
Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Babies newsletter.
RELATED VIDEO: Hilary and Haylie Duff Reveal Their Favorite Holiday Traditions!
Though it’ll be years before Luca and Ryan are old enough to join festival-goers, they’ve recently reached some age-appropriate milestones of their own. For Ava, it has been “transitioning to big-girl toothpaste.”
“It has been a real fight in our house because she really likes the baby toothpaste,” Haylie explains. “We had a couple of matches about that, but now we brush our teeth with big-girl toothpaste.”
A big success for Hilary and Luca has been getting him to sleep the whole night in his own room.
“He would always come into my room in the middle of the night and be scared, which is so sweet,” Hilary says. “I love those cuddles more than anything, but now he stays the whole night in his room and he’s really proud of himself.”
A post shared by haylieduff (@haylieduff) on Oct 5, 2017 at 8:44am PDT
Since Luca started kindergarten earlier this year, Hilary says she has started to notice big changes in his behavior.
“In two months, he’s a different child,” she explains. “He has always been a great kid, but the things he’s interested in now changed so fast.”
And the things he’s not interested in have changed, too. “He won’t eat pizza!” the Younger star says with a laugh. “Ryan wanted Luca’s pizza last night. She was like, ‘Why don’t you like that? You’re crazy.’ ”
FROM PEN: Learn Alex Guarnaschelli’s Ganache Secrets Before Your Holiday Parties
As the holidays approach, Haylie says she’s the organizer who gives “everyone specific jobs” when it comes to preparing — and first up is hosting Thanksgiving at her house.
“Haylie just sent us the text message,” Hilary says. “She’s like, ‘Hilary, you’re getting the sweet potatoes. Then she’s like, ‘Mom, you’re getting this. Please omit eggs from gravy, Hilary says no floating egg in gravy.’ I was like, ‘What does that even mean?!’ ”
Adds Haylie, “Everybody got their job, and then I was like, ‘Cool with that?’ ”
A post shared by haylieduff (@haylieduff) on Nov 9, 2017 at 9:34pm PST
When it comes to the tradition of making their grandmother’s stuffing recipe on Thanksgiving, though, Haylie means business.
“I think, for me, that’s the only thing I care about at Thanksgiving,” Haylie says, adding the key to what makes it special: “There’s a lot of chicken broth in it and the bread is dried out for two days. So it’s really kind of stale, but then it gets moist from the chicken broth.”
“It’s a long process,” adds Hilary. “But really good.”
When the two sisters can’t be together in person like they will be for the holidays, they keep a constant game of Words With Friends 2 going to keep in touch.
“I’m always in New York filming, and we’re always on different schedules,” Hilary says. “A lot of times we’re texting through the app. I’m such a gamer, and she always beats me.”