Twin brothers Dan and Mike Friedman were just 11 years old when they received the news that would change their lives forever. Their father, 44-year-old Andrew Friedman, had been inside the World Trade Center’s North Tower when a hijacked plane struck the building on Sept. 11, 2001.

“I thought he was gonna be fine. I thought he was gonna get out of the smoke, out of the fire, out of the damage and I would see him at home,” Mike, 28, tells PEOPLE of his father. “As the days went by, I started to realize what really happened. He didn’t make it out. It was a really scary thing. It was a really sad time.”

Andrew had been working on the 92nd floor of the North Tower, Mike says. He was able to call his wife, Lisa Friedman Clark, and assure her that he “had plenty of air” and was expecting to make it out alive. The last thing he told Clark was that he loved her. For days, the family searched local hospitals and met with police as authorities worked to identify the thousands killed in the attack.

Dan says he and his brother had to “mature quickly” after the death of their father — “It really changed how we had to grow up,” he tells PEOPLE.

On the 15th anniversary of the tragedy in 2016, the brothers set out to honor their father and organizations that helped them to cope in the wake of Andrew’s death. The twins recalled the first few days after 9/11, in which they provided socks to members of rescue teams who helped find their father’s body.

With that, Dan and Mike knew just how to pay tribute to their father: they decided to sell socks.

“In the back of our mind, socks had always been there. It’s the first thing that came to our minds. It’s a full circle of a story,” Dan says. “It’s one we’re really proud of and excited to be working on.”

A year later, the brothers launched TallOrder, a sock company for men with sizes 9 through 20. At 6’9″ and 6’11” respectively, Dan and Mike set out to “fill a hole” in the men’s sock industry, catering to men with large frames. The socks come in numerous patters and colors and their logo features a pair of towers.

“We were both working in corporate America … but we realized that we wanted to do something that provided a product that we needed,” Mike tells PEOPLE. “Something with more mission behind it. And we wanted to do something to pay homage to our dad and the organization’s that helped us out a lot after 9/11.”

So, the duo teamed up with Tuesday’s Children, a non-profit organization that supports families impacted by the 9/11 attacks. The brothers have promised 10 percent of all TallOrder proceeds to organizations like Tuesday’s Children.

Since the launch of the sock company last year, they’ve donated $20,000 to the New York-based charity.

“Tuesday’s Children was founded right in the aftermath of 9/11. There were people who pulled their resources together to more or less look after the children of 9/11,” Mike says. “They provided a ton of great resources for us growing up.”

Their efforts have provided the brothers with a sense of purpose, and they plan to keep TallOrder going for the foreseeable future. Dan and Mike say they know their father would be proud if he could see them now.

“I think he’d be especially proud for two reasons. One, because he’d be happy to find socks that fit, he was a big guy. He was 6’5.” And two, he’d be even happier at the fact that we honor some of his closest friends with the names of the socks,” Dan says.

Mike adds: “The most important thing he talked about with us was having fun and making every day mean something. This is what we’re doing and we’re having a lot of fun doing it … we’re enjoying ourselves. That’s something he’d be extremely proud of.”