Why 'childlike empress' and not just 'empress'?

In the book, she is referred to as the "childlike empress."

In the first movie and most of the book, she was refered to as "Child-like Empress". At the end of the first movie/first half of the book, Bastion had to give her a name to end the Nothing. It's hard to hear the name he called out in the movie but the book clarified it, the name was MoonChild. The second half of the book, the name goes back and forth between MoonChild and Child-like Empress.

In the original book, she is called the Childlike Empress. The reason why she was called that is because she is actually somewhat immortal, or at least never ages. She ruled Fantasia under many names, which was why she needed a new name in the first movie.

She's ALWAYS been "The Childlike Empress", even in Part One, and the whole source material. It's her title, since her 'power' in essence is purity and innocence, corruptible and timeless. She will always have countenance, grace, and a righteous heart. She's essentially a Goddess of Love / "Good" / Light, depending on how you want to look at it.

Cause I ride like Kelly Bundy, Yo I keep that *beep* nasty~ (Spank Rock)

Wikipedia says she's a "Goddess of Fantasy" within that book series / "World" / paradigm. The Childlike Empress (Die Kindliche Kaiserin in German) is the monarch of Fantasia, who resides in the Ivory Tower palace in the heart of the realm. Although she is nominally the ruler of Fantasia, she does not command a political system nor does she enforce her authority, and in fact rarely interacts with the outside world. In actuality, she is the living embodiment of Fantasia's life force; should she die, Fantasia and all Fantasians would cease to exist.
The amulet known as 'AURYN' is her emblem, and those who wear it are her messengers and representatives. As explained by Morla the Aged One, her lifetime is not measured in years or in time ("she's much older than the oldest inhabitants of Fantasia, or rather, she's ageless"), but by names. She continuously needs new names, which only the imagination of a human child can give to her. When she begins to need a new name, she begins to fade away, causing the Nothing to appear in Fantasia. She sends Atreyu on the Great Quest, which brings Bastian Balthazar Bux to Fantasia, and Bastian gives her the name of his mother, (according to his dreams) Moon Child, which restores Fantasia and begins the second half of the novel.
Her description is that of an indescribably beautiful young girl, appearing no older than ten, yet known to be much older than the oldest Fantasians. Her hair is snow-white, as is her gown, and her eyes are the color of gold, earning her the official title Golden-Eyed Commander of Wishes (Goldäugige Gebieterin der Wünsche in German).

Cause I ride like Kelly Bundy, Yo I keep that *beep* nasty~ (Spank Rock)

I do believe nobody gave you a straight, short and correct answer. "Because she's always been called that" is not an explanation imho, sorry everyone. I realise your post is years old but for the benefit of anyone else reading this recently:

She is called 'childlike Empress' because her form is childlike. She is the living embodiment of Fantasia, and has been since its creation. This means she is countless years old, she just looks like a child.

~It's as if she evaporated... straight through the wullz~

I think they did refer to her at least once as the "Childlike Empress" in the first movie. I have read the novel for the record, and in the novel she is almost exclusively referred to as "Childlike Empress".

It actually makes more sense to drop the "Childlike" in her name since the actress doesn't look like a kid at all in this movie, and even less so in the third movie.

Because Warner Bros wanted to tap into the paedophile market. It's a large demographic, hence big bucks are to be made.