I am aware that this question may be on the verge of Offtopicland, but I am very curious about something regarding these two numbers. Here's the story...
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 of the two numbers.
We (the Spanish speakers) are not aware of that (as far as I know), so I would like to ask any Spanish language learner present here: do "seis" and "siete" really have such a phonetical similarity as to pose a challenge to you?
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.