I want to create a successful side hustle for my son to build generational wealth.

In June, Alexandra Rutkay's side hustle became viral while she was on a movie set.

On the set of "A Complete Unknown," a biopic about Bob Dylan, Rutkay, a full-time makeup artist, worked a 16-hour day. As she clocked out that night, she discovered that her social media post had gone viral.

Rutkay, 41, sold out her New York-based side hustle Citymouse's inventory of more than 500 bags within three days after two glowing reviews of her fashion-forward mini-diaper bags gained views on TikTok and Instagram.

She quickly purchased 1,000 more bags from her Chinese manufacturer, and the orders continued to pour in. Citymouse has generated over $635,000 in sales this year, with an average of $53,000 per month, mainly through its Shopify website, as per reviewed documents by CNBC Make It.

DON'T MISS: The ultimate guide to earning passive income online

Rutkay, her husband, who recently left his job to work at Citymouse full-time, and a part-time assistant are the only employees of the company. The couple will pay themselves a combined $150,000 from the side hustle this year, she estimates. Rutkay also works up to 40 hours per week on Citymouse, which remains a side hustle because she still devotes more time to her makeup artist job.

Since dropping out of college and moving to New York in 2004 to pursue a makeup career, she has had side hustles. However, turbulence in her life upped her sense of urgency: between February 2020 and February 2021, Rutkay gave birth to her son, her grandmother died, and she was diagnosed with a rare form of cancer in her right thigh.

She is now cancer-free, she says. Despite this, the thought of her son growing up without a mother motivated her to change her financial goals. No longer content with a decent paycheck, she wanted to build generational wealth she could pass down. Entrepreneurship was her first thought.

"Rutkay admits that the fear of leaving her son without knowing her chilled her to the bone. She wonders, "If I had passed away before [my son] got to know me, what would I have left him?""

From $5,000 in credit card debt to profitability

The idea for Citymouse came from Rutkay's personal frustration while getting ready for dinner with her family in Manhattan. She realized that her Saint Laurent crossbody bag was not big enough to hold her son's snacks, wipes, and emergency pull-ups.

Meanwhile, her diaper bag was bulky and cumbersome, making it unsuitable for a dinner date.

She decided to design a smaller, more fashionable diaper bag specifically designed to fit a toddler's necessities while walking home. She found a manufacturer through Alibaba, an online marketplace, and paid $8,000 to order samples.

Rutkay sold her first 500 crossbody bags, made from shiny regenerated nylon, after more than six months of effort. Sales gradually increased as she added interchangeable straps and hot pink bags before the "Barbie" movie's premiere last summer.

Rutkay has spent $25,000 on Facebook ads this year to maintain Citymouse's momentum on social media.

She estimates that 90% of her business comes from social media referrals, with approximately 32,600 followers on TikTok and 16,000 on Instagram.

Rutkay states that the business achieved profitability this year due to the high volume of sales.

Working toward generational wealth

Citymouse offers crossbody bags at $78.99 each and mini backpacks at $179.99 each, with Rutkay hoping that end-of-year holiday sales will help Citymouse surpass $1 million in annual sales this year.

Although the global diaper bag market is worth an estimated $747.1 million, according to Grand View Research, Rutkay isn't particularly focused on becoming the top company in the industry.

Her goals are personal: She aims to earn $600,000 annually for herself and her husband through Citymouse or sell the company for at least $10 million and start a new venture.

To achieve quadrupling Citymouse's profits, a small business would require years of 90-hour workweeks. However, for Rutkay to quit her makeup career, she would need to earn $600,000 annually.

Rutkay says, "I could literally be sleeping and making money."

To earn extra money online, sign up for CNBC's course on passive income streams, starting tips, and real-life success stories.

Sign up for CNBC Make It's newsletter to receive expert advice on work, money, and life.