"It’s a very sweet spot for me," the 60-year-old from Liberty Township, Ohio, tells PEOPLE. "I know it's hard for parents, I don’t ever want to minimize that, but there's so much joy and so much you learn from raising a kid [with a disability]."
Harmon was diagnosed with the genetic disorder that causes progressive muscle degeneration and weakness at just 3 years old. Together, the mother and son did their best to make the most of everyday activities.
When Harmon was 20 and had returned home from his last year at summer camp, his mom took him to a local crafts store to find stickers to decorate a photo album.
"We were in the store looking at stuff, and I just was joking around when I said, 'Darn! There aren't any stickers here of kids in wheelchairs at camp!' " Nuenke says. "And a beautiful young woman came around the corner and she said to me, 'There aren't any stickers of kids with disabilities. Somebody needs to do that.' "
So the mother and son accepted the challenge. Nuenke drew a series of stickers featuring children with various disabilities and Harmon provided ideas and feedback. She then turned her drawings into puzzles, coloring books and bookmarks that Nuenke would give away to anyone who asked.
The project not only created awareness for kids with disabilities, it also allowed these children to "feel seen," Nuenke says.
After her artwork caught on, a local hospital asked the mother-son team to create a children's book about Duchenne muscular dystrophy.
"I think one of the big things [about the diagnosis] is parents are heartbroken and kids ask, 'What's wrong?' " Nuenke says. "Because they're 3 or 4 years old when they're diagnosed, they don't know."
Nuenke and her son worked on the book together during the last two months of his life – Harmon died from complications from Duchenne at 25.
"While he lay in bed for the afternoon, I'd bring my table up there to do my drawings and he helped me with the words. He was extremely proud," Nuenke tells PEOPLE. "We just had the best time writing that book."
The finished product, DMD and Me, is available online and offers a heartfelt explanation of what it means to be diagnosed with the disorder.
Nuenke, whose story was also covered by The Mighty, has received requests for personalized coloring books from around the country. The substitute teacher's aide sends the books – free of charge – in the hopes that they'll provide happy memories for the families receiving them.
"Chris was my only child, so I'll never have any grandkids – it's like I get to have a connection with people," Nuenke says. "Once you go through this life with a child with a disability, you're part of that no matter what."
Nuenke adds, "Most of my joy was because of Chris. This [project] is giving me so much joy right now. It's funny because I wanted to give up. I'd say, 'Nobody likes 'em,' and Chris would say, 'Mom, keep doing it!' So he must have known. This lets me stay connected."