At first the 80-year-old great-grandmother thought they were selling tickets for a draw, so she started to rummage around in her purse for change.
Then a little girl stepped forward to hand her an envelope. Inside was a $50 gift card.
"And she said, 'It's for you.' I couldn't believe a $50 gift card, passed to me for no reason. I was just overwhelmed. The tears were coming, and I wanted to give them all a hug. I was trembling," said Campbell.
Before she could get their names, the kids were gone. Campbell wanted to find them to thank them properly, so she set out to track them down.
The children and their parents belong to a group called Keeping Fit Exercise. But exercise isn't all the group does. It raises money to help people in the community when there's a need.
Members quietly band together to buy things like groceries, gas, clothing, pay sports fees, and even graduation expenses. And up until now, they have never had any publicity.
"The motto of our group is people helping people," said group leader Rachel Parise.
"We started doing this about seven years ago. We do it because we want to make a difference. Once you get it, that it's better to give than to receive, that's just a part of your life. We have always tried to teach our children that that is what it's all about."
Woman joins in memory of her son
Marie Myers is a long-time member of the group. She says her son T.J. was a young man who really understood the power of giving back. When he died four years ago, Marie wanted to find a way for his spirit to live on.
"T.J was a very kind soul, who always tried to make someone smile or laugh," said Myers.
"The night of his passing, my husband and I promised that we would continue that. We just do what we can to help people, and make them smile, and it makes our hearts smile."
Wanda Johnston and her 11-year-old daughter Leah are also part of the Keeping Fit Exercise group.
On Leah's birthday this year, she told her mother she didn't want any presents, she only wanted to celebrate through doing random acts of kindness.
"We gave away gift cards, and we paid for people's groceries," said Leah.
"It's a special feeling that you never get any time else. They were smiling so much, and it just made me feel so happy."
"I'm pretty proud of her," said her mother.
"She's always been a child who looked out for others and wants to be there to help out. It just makes me honoured to be her mom."
For Campbell, that trip to the grocery store is one she'll never forget. She says that little act of kindness made her feel honoured.
"I have been telling everybody what these kids have done, and it's just a beautiful story," she said.
"I love them all, I'm going to call them all my family now."
Next week Campbell will be hosting a party at her home for all of the members of the Keeping Fit team. She has already knit everyone mittens, and made lots of sweets.
She said she wants the kids to feel the same joy they gave her.