pronunciation of 哪 is "nǎ" while 儿 is "ér". But when these two words is put together, they combine to one pronunciation "nǎr", not two words sound. you can take a look at here for a reference: https://www.youtube.com/watch?v=sI-2T2ghl9Q
If you want to remember the characters, that's easy. 镜(mirror) has 钅(metal) on its left side. Before the invention of glass mirror, mirrors are made of polished bronze. Anyway, a character with 钅 is not likely to represent a human body part.