Doug Dunlop's son, Tadhg (pronounced Tige) had $200 of his own money on Sunday and was looking to add to his vast Lego collection.
While he was there, he was asked his age. When he told the store manager he was 11½, a security guard was called and the boy was asked to stay in a certain area of the store until his father arrived.
Tadhg had been dropped off at the mall by his father and the two were planning to meet later for lunch.
The boy had shopped at the store alone many times without incident, said his father, and had spent thousands of dollars there — money he made shovelling snow and babysitting.
Tadhg had even been asked his age before on several occasions, and the store employees would joke that it wouldn't be long before he could work there.
Dunlop was upset to discover his son had been detained in the store.
"I was, of course, shocked," he said.
When he walked into the store, he thought that Tadhg might have knocked over a display, but not that he had done anything wrong, said Dunlop.
"Why would you detain a good customer? It's bad business."
'Utmost concern is for child safety'
The store policy was not visible inside the store at the time. But Dunlop, who posted his letter to Lego in a blog about the incident, asked the store manager why the policy existed. The store manager cited safety concerns such as child predators or the possibility of an evacuation from the mall.
Dunlop said he was called a "bad parent" while questioning the store manager and the security guard about the policy.
As of Wednesday, the store has posted a sign in the window that reads: "To ensure that your child has a safe and enjoyable experience in our store, please do not leave them unattended."
Amanda Santoro, a Lego brand relations manager, told CBC News in an email the company stands by the policy.
"As a toy company, our utmost concern is for children's safety, and as such we have a policy in place regarding unaccompanied minors. As this customer was under the age of 12 and alone, we followed our protocol and stand by our policy."
Dad calls policy 'age discrimination'
In a lengthy letter to Lego, Dunlop says he wants an apology from the store for detaining his son and for being called a "bad parent" by letting him shop alone.
Now the Dunlops say they will never shop in the Calgary Lego store again.
"My son loves Lego. It's his favourite store," he said. "This policy doesn't make sense; it's arbitrary age discrimination where they decided that children under 12 are inconvenient."
Tadhg has also been telling his friends about the incident, said Dunlop. "And they are incensed. Being 11 years is not a crime."