Did they have more children and if so , what were their first names?
So there's a difference of about 3 yrs in the birth year were you come with your dates and the Jan I found born in 1820
Problem is that I don't know how to find out for sure if that's the right one,
first name of father is Jan (John) just like you wrote,
his parents are both deceased when your John marries, so that all fits.
But....what to do with those 3 yrs difference....
I found a brother and sister of Jan born 1820, so that's why I ask for the first names of the other children.
I don't know if it happened that when one came in Australia the age was not quite correct given or that it was the right age (and so birth-year)
The Jan born 1820 lives not in Amsterdam anymore in 1851 according to the Population registration of 1851-1853, so he left Amsterdam for sure.