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In a recent argument I referred to a person as being the "youngest child". I was corrected that she was the "younger child" because there are only two children.
I had not heard this before and would like to know who is correct
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If there are only two children then I would use; the younger child as it is the comparative form (comparing two things)

We use the superlative when we compare a number of things; I have the biggest feet in my class. (In other words, there are a number of people in my class)
Comments  
Students: We have free audio pronunciation exercises.

Rafael is the taller or tallest pupil in our class

Tallest