“Maybe that Hashem only gave them one child.”
The Goq is clearly discussing people chose to have one child quite intentionally; one does not usually applaud people for things they have not intentionally done. (Also, if having only one child was not intentional there’s no “restraint” to be applauding.)
“How unfair and unkind to make fun of such people.”
Nobody’s “making fun” of anybody. We’re simply pointing out that intentionally having only one child is to condemned, not applauded.