This is a variation of this same question in the Spanish Language site, as I was told it was probably better suited here.
When I was learning Japanese (a long time ago in a galaxy far far away), my Japanese teacher told us that she had a hard time trying to tell apart the numbers "seis" and "siete" when she was learning Spanish, because both sounded quite similar to her. The whole class bursted in laughter (as that was her main purpose, to tell a funny story), and I do not know why I kept that anecdote in my mind.
Time passed and now I am a father with a two-year-old son, and every time he tries to count to ten, he always skips "siete" and goes from "seis" to "ocho", which reminded me what my Japanese teacher told us that day. Maybe he skips "siete" because of the phonetical similarity with the previous number.
We (the Spanish speakers) are not aware of that (as far as I know), so I would like to ask: do "seis" and "siete" really have such a phonetical similarity as to pose a challenge to tell them apart?
Maybe this question is not addressed to those who speak a similar language. English also has "six" and "seven" with the same potential similarity, as have other close languages.