The event was all about encouraging kids to dream big, but Diamond Ivy and her Little Sister, Monsiea, never imagined the star of the show would be that big.

“I just thought it would be some Minneapolis performer who’s been a good mentor,” said Ivy, speaking of the surprise performer for a Big Brothers Big Sisters of the Greater Twin Cities event midday Saturday at the Fine Line Music Cafe in Minneapolis. “I didn’t think it’d be anyone this amazing.”

The unannounced special guest for the shindig wound up being John Legend. A multiple Grammy winner and maybe the smoothest man in the music business, the piano-playing R&B/pop singer not only performed a couple of songs but also goofed around with the hundreds of “Bigs” and “Littles” in attendance at the downtown Minneapolis nightclub.

Legend was recruited by American Family Insurance, which picked Minneapolis as the fourth city to host an installment of its charity campaign “One Saturday to Dream Fearlessly,” for which different celebrities and charities are chosen in each city.

Pat Cooper, board vice-chair of the local Big Brothers Big Sisters chapter, was certainly thrilled about the choices the insurance company made here.

“This is a wonderful experience for our Littles who are here, but it’s also very beneficial to the organization on the whole to help raise awareness for our great cause,” said Cooper, who pointed out that about 2,100 children are currently being mentored locally via BBBS — but there’s a waiting of around 700 more.

The mostly preteen youths who made it out to Saturday’s event got to feel what it’s like being on a VIP guest list. They filed in for free food and drink just before noon without knowing who would be taking a seat at the piano on stage. Screams went out when Legend took the stage, where he talked about his ambitions as a kid growing up in Springfield, Ohio.

“I dreamt of being on the Grammys. I dreamt of performing with Stevie Wonder,” Legend told the young audience. “The path of dreaming it and realizing it took a lot of work, and it took a lot of people believing in me — just like your Bigs and your community believe in you.”

Legend then mixed it up with kids at different activity stations in the club, including a music-making set-up with guitar, drums and keys, and a virtual-graffiti gadget where he signed his name in fake spraypaint on a video screen (“Not much better than the real thing,” he quipped of his messy autograph).

One of the Bigs who hollered with delight as Legend appeared, Brett Stewart of Bloomington did have a big name in mind beforehand for who the unannounced guest might be: rapper Kendrick Lamar, due in town to perform at Xcel Energy Center on Saturday night. The true surprise was huge to Stewart, who has the music notes and words to Legend’s song “Dreams” tattooed on his arm.

“John is the epitome of someone who made it big with drive and determination, and who hasn’t lost touch or gotten vain once he got famous,” said Stewart, who earned some playful prodding from his Little, Michael, over his excitement.

“I’m a bit of a nut for him, so I screamed pretty loud,” Stewart laughingly conceded. He and the rest of the crowd later sang along loudly as Legend delivered his recent hit, “All of Me.”

While Diamond Ivy of Minneapolis talked about how much the singer’s appearance meant to those active in the organization — “It makes us all feel even better about what we’re doing,” she said — her singing partner, Monsiea, had one of Legend’s famous friends in mind.

“I want him to say hi to Beyoncé for me,” the grade-schooler said.

Hey, nothing wrong with dreaming of the biggest of the big.

@ChrisRstrib