That's just the way the word שן turns to plural, it's irregular. שן (shen) ==> שיניים (shinayim)
Shouldn't we tell them that שיניים refers to the two sets of teeth in our mouths? Not just two teeth.
Or does it also refer to only two teeth? Which leads to the question: How do we track a baby's tooth-growing progress numerically?
It can refer to two teeth. Just like נעליים or גרביים (shoes, socks), the only plural the word שן has is the irregular 'double' plural, שיניים.