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Over 37 Million Hits!

Sophia Grace Brownlee, 9 & Rosie McClelland, 6

Hometown: Essex, England

Since last fall's cover of Nicki Minaj's "Super-bass," the cousins became regulars on Ellen and joined Taylor Swift for tea. "Rosie doesn't understand what fame is," says mom Danielle. "But she knows girls scream as they walk past."

Over 9 Million Hits!

Makena Fedorick, 3

Hometown: Grand Prairie, Alta.

"Our phone rang off the hook. It consumed our lives," says Heather Fedorick of the reaction to her daughter's take on Adele's "Someone Like You," posted in March. As for the toddler's sophisticated song choice, "We don't listen to kid music," says Heather. "There's no 'Itsy Bitsy Spider' in my car."

Over Half-a-Million Hits!

Grace Youngberg, 3

Hometown: Fair Lawn, N.J.

The video of Grace rocking out to Gotye's "Somebody That I Used to Know" caught the attention of Jimmy Kimmel and Kathie Lee Gifford, but her biggest fan is her 10-month-old brother Connor. "She gets in his face and sings," says mom Meaghan. "She'll take any audience."

Over 4 Million Hits!

Justin & Jeremy Garcia, 6

Hometown: Millbrae, Calif.

Their adorably in-sync videos-including last year's re-creation of Bruno Mars's "The Lazy Song"-landed them a Nintendo Wii ad that aired during the Grammys. But the twins have no desire to go pro. "They're starting first grade in the fall," says mom Nina.

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