I think the voice fits each one of those you've just cited.
Well, Black Star acts like a child, sounds like a child, his voice should resemble a child, don't you think? He's in a development, like, it took about 9 editions for he to conquer his first soul, and Maka took about 2 pages lol. (no, no flashbacks)
Soul Eater's voice too, is fitting. White hair always had a meaning in the anime of " experience ", even though Soul doesn't have much, but he doesn't act like an infant in a fight like Black Star Does.