At first, all you see is the swish of an iridescent tail and a flash of red hair.

She glides gracefully through an underwater world of fish, coral, sea turtles and shipwrecks. The familiar twinkle of a Disney song settles in, and as she swims to the surface light, the princess is finally revealed.

“She’s Black?” asks Ke’Iona Shanks, who turns 8 next week. “Yay!”

Ariel, portrayed in Disney’s previous film as white with fire-engine-red hair and blue eyes, now has red locks and is Black.

Ke’Iona’s reaction was one of many moments captured by parents on TikTok as they showed their Black daughters the trailer for a new live-action version of “The Little Mermaid.”

Ariel is played by Halle Bailey, one half of the Grammy-nominated R&B sister duo Chloe x Halle. Her role signifies one of the few times a Disney princess has been played by a Black lead. The reaction videos capture the moment young Black girls watch the trailer, many in their pajamas, and for the first time see a Disney princess who looks like them.

“Seeing an actual manifestation of something you love and are passionate about makes that thing more realistic and makes you feel included; that’s my hope for them,” said Sterling Shanks, who recorded his daughters — Ke’Iona, Lai’Anna, 7, and Ea’Iona, 2 — as they watched the trailer. “Seeing Halle Bailey as Ariel makes something they love more attainable and relatable.”

Mr. Shanks said their reaction brought him to tears.

Mr. Shanks and the girls talked about the trailer for hours after they watched it on Saturday morning. “I just know from conversations we’ve had with them that in their own way they wish they had representation in the things they watch and do,” he said. He added, “Lai’Anna said, ‘Did you notice she has braids like we do sometimes?’”

Ms. Bailey was not made available for comment, but on Twitter wrote that she was “in awe” of the overwhelming response to the trailer. The movie is set to come out in May.

“I want the little girl in me and the little girls just like me who are watching to know that they’re special, and that they should be a princess in every single way,” Ms. Bailey told Variety in August. “There’s no reason that they shouldn’t be. That reassurance was something that I needed.”

The joyful response to the movie was a sharp turnabout from the racist backlash Ms. Bailey faced when the casting for the film was announced in 2019.

In Disney’s nearly 100-year history, there has been only one Black Disney princess — Princess Tiana in “The Princess and the Frog,” a 2009 animated feature starring Anika Noni Rose. The singer Brandy starred in a 1997 made-for-TV film version of “Cinderella,” a remake of the Rodgers and Hammerstein musical.

Will Fleming, whose wife, Dariana Fleming, filmed their daughter Rylie, 2, watching the trailer, said he had admired the rise of the Bailey sisters, who gained fame after covering popular songs on YouTube. When their version of Beyoncé’s song “Pretty Hurts” went took off on social media a decade ago, the superstar took notice and signed them to her Parkwood Entertainment label. Both sisters appeared on the series “grown-ish.”

For Mr. Fleming and many other parents who remembered Ariel as a household name from childhood, this reinvention is particularly meaningful.

“The fact that she rose from social media, posting online, to now being the star of one of the most classic Disney movies and being a Black woman in America — to me there are so many things to pull from that,” Mr. Fleming said. “Unfortunately some folks are looking at it like: Well, the original movie wasn’t a Black woman. I think we’re missing the point. It’s necessary, and I’m especially grateful that Disney took the chance to do this, even though they probably felt they were going to get backlash. It’s so important for every generation coming up to see this.”

Shannon Lanier said he was “very excited” when he heard Ms. Bailey would be playing Ariel “because she is a woman of color and I am raising two girls and a little boy of color, and it is so important to have representation so they can be proud of who they are, and be able to have the ability to imagine that anything is possible — even if it is a fairy tale.”

“She’s Black! Oh, my God!” yelled his daughter Madison, 11, wide-eyed and smiling, in the video recorded by Mr. Lanier. “Yes, yes, yes!” her brother Carter, 6, chimed in. “I can’t wait to see this!” added McKenzie, 9.

“It is really exciting to have a mermaid of color because it inspires probably more people of color to try things,” Madison said in an interview. “I thought it was really cool because I had never really seen that.”

Carter said he was still waiting for a Black prince.

Nicky Lungaro saw one of the reaction videos on TikTok and thought she should try the same thing with her daughter Maya Everage. Maya, 9, sat in front of the television in her pink onesie with her two braids dangling down her back. When Ariel appeared, she was speechless, looking back to Ms. Lungaro as though she couldn’t believe her eyes.

“She was incredibly excited to be able to see herself on the big screen,” Ms. Lungaro said. “This is way past due. This shouldn’t be that big a deal. This shouldn’t be that momentous an occasion, but it is, because Black and Indigenous people of color are so underrepresented in media. It’s their time to shine.”

Maya, whose favorite Disney movies include “Pocahontas” and “The Princess and the Frog,” thought the new Ariel “was really beautiful and has really long locks.”

“She looks like me,” she added in an interview.

Ms. Lungaro said she was amazed by how “a sweet moment” could have “such an impact.”

“It’s so incredibly beautiful and overwhelming,” she said.

Precious Avery, who filmed her 3-year-old, Emery Garcia, watching the trailer, agreed.

“I just love that she’s able to see someone who looks her,” Ms. Avery said. “She loved the movie before, but the sparkle in her eye that I see when she saw the trailer verified how much representation really matters.”

Ms. Avery is taking Emery to Disney World soon and had thought about what might happen if they saw Ariel at the theme park. “Now this is what she thinks Ariel looks like,” she said. When Ms. Avery brought it up, Emery said she didn’t think Ariel would be there anyway — because she lives in the water.

Mr. Shanks is also headed to Disney World with his wife, Meredith, and their three daughters. He, too, has thought about the conversation and said he would tell them that the movie was not out yet so Black Ariel would likely not be there. “They love all Disney princesses,” he said. “I think it helps them to know that they will be included in being princesses.”