"In act 20 of the manga, it's stated that she is chronologically 900 years old, which lends credibility to her apparent wisdom, maturity, and rather serious personality over her teenage biological mother Usagi (in the 20th century). Her youthful appearance attributed to the fact that her physical body inexplicably halted its natural aging process when she was five...In the original anime, she is more judgmental and does not mature as much as her manga counterpart. But she does in Sailor Moon Crystal and gets along better with Usagi." -Chibiusa Wikipedia

Take Sailor Chibi Moon's personality with a grain of salt. Ultimately, her voice should be the youngest sounding out of them all, but not be baby-like. 

You do not have to copy the performer's voices, but rather just mimic the essence of the character's voice. Sailor Moon should be cheerful and bright, Neptune should be calm and mature, etc.

Please use the mic you intend you complete this project with if you get the part. Have your audition be acapella, but in time with with the song. Use the video above to reference the song.