...right out of high school? Michael (https://goldengirls.fandom.com/wiki/Michael_Zbornak) and Kate (https://goldengirls.fandom.com/wiki/Kate_Griffiths), the only two mentioned were not old enough to be that child (Dorothy and Stan were married for 37 years and presumably, Dorothy was a year or two post-divorce when the series started).

...right out of high school? Michael (https://goldengirls.fandom.com/wiki/Michael_Zbornak) and Kate (https://goldengirls.fandom.com/wiki/Kate_Griffiths), the only two mentioned were not old enough to be that child (Dorothy and Stan were married for 37 years and presumably Dorothy was a year or two post-divorce when the series started).

Kate was 40 or 42 in the series.

We had to assume she was, yes, but neither actress who played Kate looked anything close to 40.
If Dorothy and Stan were married 38 years, working backward means they were married in 1946 and divorced in 1984, about a year prior to the start of the series. So Kate was probably born in 1947, making her 38 at the time she got married in the second episode. Certainly possible from the first actress who played her but definitely not the second.