She was arguably fully grown. Her "costumes" to me looked like the clothes you would see on a doll, child or Peter Pan. In any effect the costuming for Kes would have been just as apropos for an episode of "How to catch a Predator" that she wasn't just being made to look unique but much much younger than she really was.
Although pigtails and a lolly pop might have been too much?
With how her brain worked, she could have gotten through a simulated version of Starfleet Academy in a few weeks, and then she could have been wearing a uniform too like the rest of the crew, a 1/4 of which who totally did not deserve their rank and privileges and in no way earned their uniforms, including Kim.
Although pigtails and a lolly pop might have been too much?
With how her brain worked, she could have gotten through a simulated version of Starfleet Academy in a few weeks, and then she could have been wearing a uniform too like the rest of the crew, a 1/4 of which who totally did not deserve their rank and privileges and in no way earned their uniforms, including Kim.