That's like saying "child" can also mean "boy" and that "bird" can also mean "pigeon".
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”.
Exactly. Similarly to how languages like Spanish have implied gender, Hawaiian often has an implied male leaning unless otherwise specified like with kaikamahine. Mahalo!
The ā is a long vowel like in Latin. This would be similar a father in English. The ah in father is a long vowel. So, instead of saying kane quickly, say the ā longer like in father in English. I hope that helps.
I need to know why "boy" needs to have keiki in front of it. I know that kāne means man, so is boy literally "child man"? Also, why doesn't this apply to kaikamahine?
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