Kafei was turned into a child by Skull Kid, while he was still under the control of Majora. Therefore he is a grown man in a child's body. So while his thoughts are adult-like, his voice and body are not. With that being said, when deciding what voice to use, imagine an adult with a child's voice. He sounds mature and he sounds like he knows what he's talking about, but his voice is that of a twelve year old. I don't want his voice to be very high pitched though.