First of all, "little darling" may be used in some situations, for example, in some places or in some cases, people may use "little darling" to address particularly cute or obedient children. Therefore, when Xiaobei and Hanazono Sakura are together, they may be called "little girls" to express their intimacy and loveliness.
In addition, "little darling" may also be used in a specific culture or background. For example, in some cultures, people may use words like "little darling" to address their children or younger generation to express their love and concern.
In a word, according to the information you provided, the specific meaning of "little girl" may have many explanations. If you can provide more background information, I may be able to give a more accurate answer.