Last night's Halloween carnival-themed party at the Little Red School House in New York City's Greenwich Village was packed with children sporting her new collection.
And the designer seemed both relaxed and excited at the scene, which included a bouncy house, games and face painting.
"You're a little tricker," she said to a towheaded boy, as she tickled him and he erupted into laughter.
"The military jacket is sort of the star of the show," she told StyleList, surveying the several boys in the room wearing it.
"If you take it up to the larger sizes, I think we can squeeze into it. I know I'm going to," the designer admitted to us.
McCartney started out creating the capsule collection by making things she liked, but that quickly changed.
"Halfway through the process, my eldest son, who is four and a half, started having a mind of his own in what he wanted to wear. All of the sudden, I thought, my God, I have to take this to a different area where I'm trying to dress kids," she revealed.
"Part of it was quite weird -- I realized I would have to figure out what teenagers will like, so we tried to cover the options."
Still, she liked the process enough to entertain adding a kids' collection to her Stella McCartney label.
"We'll see how this goes down," she told StyleList. "This is not something we've really thought about, but we'll see how people react to this. I'm really intrigued to know how it works. "
It was easy to tell what the best item from McCartney's Fall '09 ready-to-wear womenswear collection was -- cropped charcoal trousers with a black contrast waist and side stripe -- worn by McCartney herself and Vogue's Virginia Smith (who happens to be married to Gap's executive vice president of design, Patrick Robinson).
While McCartney's three children were not in attendance, it was still a family affair for the designer, as her father Paul's girlfriend, Nancy Shevell was there and received a warm welcome from McCartney.