Kourtney Kardashian: Why I Named My Son Reign Aston

"I actually had the name on our list with Mason," she tells PEOPLE

When it comes to naming her children, Kourtney Kardashian is on point with her choices, creating names that are on trend and meaningful.

After finding out she was pregnant with her third child, the wheels started turning to come up with a name that would compliment her first two children, Mason Dash, 5, and Penelope Scotland, 2½, with longtime love Scott Disick, 31.

“I’m never certain [with names] until they’re born,” the reality star and designer, 36, tells PEOPLE exclusively.

But once she settled on Reign Aston, it was perfection. “I love it!” she adds.

“I actually had the name on our list with Mason,” she explains. “Then with Penelope, I had it on the list but spelled R-A-I-N.”

And for her third child, which Kardashian was convinced was a “girl in the beginning,” she was once again leaning towards Rain as the name. “Once I found out I was having a boy, we just changed the spelling again,” she says. (And her 4-month-old baby boy has some pretty cool initials, R-A-D, which is a fun added bonus.)

Now back at work focusing on her Kardashian Kids collection, which launched at Nordstrom.com Monday, the eldest Kardashian sister is settling into her role as working mom and enjoying her time designing with her two sisters Kim, 34, and Khloé, 30.

“We put so much into it and were constantly sending references to each other,” she notes.

“We take a lot of pride in the line and we’re all kind of perfectionists and very detailed oriented,” she says while explaining why the line has had such success.

The collection, which is a personal one for Kardashian as a mom, reflects her own style and that of her children, and Kim’s daughter North, 22 months, who all have their own unique tastes in fashion.

“Mason is very picky with what he wears,” she says with a laugh. “I think it’s fun to let him have that expression. He’s all about comfort.”

Penelope on the other hand is similar to her mom, but very much a “girly girl,” Kardashian says.

And for baby Reign? “He has an amazing wardrobe,” one that’s chockfull of hand-me-downs from his siblings and cousin North, she admits.

For more from our interview with Kourtney Kardashian, pick up the latest issue of PEOPLE, on newsstands Friday

— Jennifer Garcia