That’s English. This is Hawaiian. From the Hawaiian Dictionary:
keiki 1. nvi. Child, offspring, descendant, progeny, boy, youngster, son, lad, nephew, son of a dear friend; calf, colt, kid, cub; worker; shoot or sucker, as of taro; to have or obtain a child; to be or become a child.
It’s not necessary to modify “keiki” with “Kāne”.
It does. Kaikamahine comes from "keiki"+"wahine". There are a couple of sound changes that take place there for which I donʻt have any particular explanation. Itʻs worth pointing out that a similar thing happens with words for parents:
"makua" = parent
"makua kāne" = father
"makuahine" = mother
If you just wrote kane it would be man. The keiki meaning child before kane meaning man changes the meaning. It literely translates to child man. So, what is a child man? A boy. Also, if you just put keiki by itself, it would mean child. So keiki and kane are like buddies. If you separate them their meaning changes, and then you will get the question wrong. ;-) Ok? Hope this helped.