Katie said Ruairi had held a season ticket since birth and regularly attended home, away and European matches with his family.
Supporting Man Utd was “all we do”, she said, adding that she met her husband at Old Trafford and had held her own season ticket for nearly 20 years, along with her late mother and siblings.
“Since [Ruari] could walk, he’s had a ball at his feet,” she said. “He’s just football daft.”
Katie said Ruairi had been a United mascot at Crystal Palace last season and had travelled to watch the team in Seville, Istanbul and Bilbao.
A post on X, in which Katie said the restriction would “break [Ruari’s] heart”, has been viewed more than 1.3 million times.
She said she had not told him about the restriction because “you can’t put that on a six-year-old”.
“The first game of the season, we’re definitely going to miss,” she said.
“I looked at his dad and said, ‘How can you take that away from him?'”