Rachel Griffin Accurso was rushing to grab pastries on her way to a recording studio one spring morning in Manhattan. She grew hot as she left the subway with the sun beating down on her, and she took her jacket off without thinking.

And just like that, she had unintentionally transformed into her alter ego.

Wearing her signature bluejean overalls, pink T-shirt and a matching headband, she became that friendly woman from the videos: the one who joyfully pronounces words, babbles if necessary, waves and sings to instruct her little viewers.

She had morphed publicly into Ms. Rachel, playfully described as the “Beyoncé for toddlers” in a TikTok comment. For many, she has become a household name as she has seen her children’s videos, “Songs for Littles,” skyrocket in popularity over the last year, garnering more than 4.8 million YouTube subscribers.

“I got stuck recording videos for everyone,” said a mildly-out-of-breath Ms. Griffin Accurso, as she arrived a few minutes late to the 10th-floor studio in Midtown.

“But I don’t mind. I love being able to help make people’s days,” she added.

Then, she was ready to bring on her charm.

She was chatting with everyone on the set, whether performers or technical staff. She asked: What time were the actors’ Broadway shows that day? What had everyone been watching on TV lately? Wasn’t it cool to be filming in a real studio instead of her one-bedroom apartment?

When the calls came for lights, camera, action, she smiled widely and her voice rose an octave.

“Can you be a crab?” Ms. Rachel cooed into the camera. “Now, let’s be a starfish!”

She made it look easy, because the work that has made her an internet celebrity for the tykes is more than a performance to her. “People call ‘Ms. Rachel’ acting, but it’s really just who I am, except a more excited version,” Ms. Griffin Accurso said. “I’ve found my calling.”

Hardly any money has been spent on promoting or advertising “Songs for Littles,” Ms. Griffin Accurso said. Though she is also wildly popular on other social media platforms like TikTok and Instagram, Ms. Griffin Accurso has the most followers on YouTube, which remains the platform where her work generates the most revenue from paid advertisements. The business became so successful in recent months that Ms. Griffin Accurso’s husband, Aron Accurso, quit his full-time job as associate musical director and associate conductor for “Aladdin” on Broadway.

The path to success

Growing up in the small community of Springvale, Maine, Ms. Griffin Accurso was never quite sure what her career path would be. But she knew that she loved children and serving people. A job working with children at a Boys & Girls Club first inspired the idea of combining those interests with music, though it would take several years for those two passions to come together.

She moved to New York City on a whim in 2009, after reading a quotation from Mark Twain about people coming to regret not going after their dreams.

She worked as a nanny and picked up odd jobs. Less than a year later, she met Mr. Accurso at a Unitarian church on the Upper East Side and found a kindred spirit.

Mr. Accurso has a distinct memory from their second date, when she asked him, “Don’t you just love Mr. Rogers?” She was referring to her fondness for Fred Rogers, the friendly television host who spread a message of kindness to generations of children.

She and Mr. Accurso pursued a collaboration, composing songs and making a musical about mental health. Ms. Griffin Accurso earned her master’s degree in music education from New York University and started working as a music teacher at Bedford Park Elementary School in the Bronx. They married in 2016 and had a son, Thomas, in 2018.

Ms. Griffin Accurso left teaching full time to be with her son. Around his first birthday, she noticed that he was behind on key milestones, particularly concerning speech. “His mouth wasn’t connecting to his brain,” she said.

The couple sought speech pathology services, but Ms. Griffin Accurso wanted to supplement his learning. Her search turned up dry, so she started making videos.

She filmed close-ups of her mouth to show the pronunciation of words and recorded her versions of children’s songs, making sure to incorporate voice, sign language and visuals. She also recorded music classes that she taught in person, and the couple posted the videos on YouTube. They figured it wouldn’t hurt if others found them helpful.

The videos struck a chord. “It makes so much sense to everyone else, but to me, it feels accidental,” she said of her success.

Teaching speech

Maura Moyle, an associate professor of speech pathology and audiology at Marquette University, said that the Ms. Rachel videos she saw incorporated key techniques that speech therapists use to help children, such as speaking slowly, saying simple sentences and repeating them.

Research shows that young children are attracted to “parentese” or “motherese” — the kind of “baby talk” that the videos predominantly feature, in which the voice gets higher and facial expressions are exaggerated, Dr. Moyle said.

“She’s getting infants to pay attention to language and pay attention to speech sounds,” Dr. Moyle said. The videos are no substitute for speech therapy or children’s interactions with adults or caregivers, she said, but they can be “a great tool to use.”

Joseph Viramontez and his wife, Kristyl Parker, struggled to secure speech therapy and other treatment for their 2-year-old daughter, Aranea. Many nights, he went to bed feeling that he was failing because his daughter was having frequent temper tantrums, and he and his wife did not understand what she was trying to tell them, he said.

Mr. Viramontez, 29, tried to come to terms with the devastating thought that he’d never hear her say, “I love you.”

Even state programs in Texas, where they lived before a recent move to Pennsylvania, were booked more than a year out and insurance was denying additional testing for autism, he said. Mr. Viramontez said his wife learned about Ms. Rachel on TikTok.

Aranea’s parents noticed a change within a month of her watching Ms. Rachel’s videos. Instead of screaming when she was hungry, she would rub her belly and use words, “blossoming with her communication,” as he put it. She has even said those three words that Mr. Viramontez had longed to hear.

From idea to video

Be it about teaching nursery rhymes, discussing emotions or helping children to talk, each “Songs for Littles” video starts with a theme. And with each theme comes a bucketful of research on related topics.

For an upcoming video about skills that teachers look for in children before kindergarten, Ms. Griffin Accurso spent weeks analyzing the requirements in various states and reading research papers. She wants to get it right, she said.

Ms. Griffin Accurso and her husband collaborate on making scripts and sketching out scenes, as well as deciding on the actors they need. Mr. Accurso edits and writes the music for the videos with the help of an outside editor, who also makes animations for them. The couple rehearse the songs, which can be popular children’s tunes performed the Ms. Rachel way, or original compositions by her and her husband, who also plays the role of the puppet Herbie.

The team wants each video to be inclusive about gender, disabilities and race. One regular performer, Jules Hoffman, is nonbinary, which caused a backlash among some viewers earlier this year. Though the negative reaction pushed Ms. Griffin Accurso to take a short break from social media, she says that she remains undeterred about representing a wide range of perspectives.

‘A lifesaver’

Brandice Elliott, 33, first heard of Ms. Rachel through a mothers’ group online. Having just returned to full-time work after maternity leave, Ms. Elliott needed time for chores. So she tried the videos, and they worked.

When Adeya, her 1½-year-old daughter, hears Ms. Rachel, her focus shifts to the screen, Ms. Elliott said. If toys are being put away in the video, Adeya will put away toys. If Ms. Rachel is belting out “The Ants Go Marching,” Adeya will march in place. And she claps along and mimics the gesture of gum being stuck to her hand to the beat of “Icky Sticky Bubble Gum.”

“Ms. Rachel has really been a lifesaver for us,” Ms. Elliott said. “When I put those videos on, I know Ms. Rachel is not only going to sing, but she’s also going to teach her.”

Ms. Elliott is even more amazed by how much she gets out of the videos. She picks up on the sign language and takes note of the tone of voice. She finds herself asking her toddler if she is hungry, as Ms. Rachel would, and she gets a response.

“Ms. Rachel is our Mr. Rogers,” Ms. Elliott said. “She is really changing how the kids nowadays learn.”

For her part, Ms. Griffin Accurso often lies awake at night, thinking about what she might be able to do to help children who don’t have access to education.

She wants to keep speaking, and singing, for the little ones.

“I never get tired of singing ‘Icky Sticky Sticky Bubble Gum,’” she said, “so I must be meant to sing it.”