But the remaining examples of the -n plural are all reduplicative. As Middle English simplified forms, sometimes two fading forms got mistakenly combined, and it stuck. The plural of child was childer, but became children. The plural of brother was brether, and became brethren. And kine conceals the same process. Before we said "cow," we said "cu" for the beast. Middle English ku had a plural, ky, which attracted the ending -n because it looked like it needed something to become plural.
In modern Northumbrian dialect the word kye remains. It is pronounced [kai]. With this lyric, you can see where English was heading before the southern dialect won out in the Middle Ages.
The kye's come hyem, but Aa see not me hinny,
the kye's come hyem, but Aa see not me bairn.
Aa'd rather loss all the kye than loss me hinny,
Aa'd rather loss all the kye than loss me bairn.