Even a two-year-old can easily write and say these three Chinese characters.
Yes, I know saying yi1, er4, san1 is a bit harder, but my American-born son could say it when he was 2.
Don’t you think it is easier than pronouncing “one, two, three”?
This is part of the “Basic Mandarin” Lessons at ChineseBay.com
You may compare “1, 2, 3” in Mandarin, Japanese, Vietnamese, French. Which one is easier?