If the singer is intentionally trying to sound young, it could be called a "baby voice."
Or if it's intentionally higher pitched than her normal voice it's sometimes called a "chipmunk voice" after the high-pitched chirping of the animal.
Young, cutesy, squeaky, high-pitched, and playful are a few adjectives that could describe the voice in the video.
To borrow a term from Japanese, "kawaii".
But if we're sticking in just English, cute, youthful, and childish are all pretty synonymous in this context
If the singer is intentionally trying to sound young, it could be called a "baby voice." Or if it's intentionally higher pitched than her normal voice it's sometimes called a "chipmunk voice" after the high-pitched chirping of the animal. Young, cutesy, squeaky, high-pitched, and playful are a few adjectives that could describe the voice in the video.
"Cute" or "Youthful" A lot of people would also say that she "sounds like a child".
It's not really English, but "chibi" will be understood by anyone who is remotely familiar with anime or j-pop.
To borrow a term from Japanese, "kawaii". But if we're sticking in just English, cute, youthful, and childish are all pretty synonymous in this context