For someone with a 1 year non O marriage extension (also have 2 children), is it recommended that to be safe one carries Bt 20,000 or Bt 40,000 cash on hand when one arrives back at suvarnabhumi, or is it not necessary?

It is very unlikely you would be asked to show the 20k baht since it is intended to be for entries with a visa.

I don't recall any report of anybody that had a re-entry permit for a long stay extension being asked to show it.

Thanks Ubonjoe for sound advice

You should — under the rules — have the 20K, but they are not going to ask to see it unless they are looking for a reason to deny entry; and there’s no reason why they would look to deny you entering with a valid re-entry permit.

 

It is, currently, only long term tourists that are at risk of being denied entry.