It would certainly be worth pointing out to them that if they did take one kitten then find it to be, well, a bit too full-on
... that adding a second young cat later to alleviate the pressure is perfectly do-able.
That gives 'em options when they discover how much hard work a single kitten is (lets be honest, this is why most young cats adopted as kittens end up back in rescue, as their owners just can't cope with the demands of a youngster).
So without pre-judging, Alison, we will be guided by your common sense and knowledge of the couple, and keep our fingers crossed for them.
It might also be a good idea to enlighten them to the existence of The Cinnamon Trust
http://www.cinnamon.org.uk/for the years ahead.
Well done, you. Don't get frustrated, they may get lucky with a perfect kitten from the Vet and live to be 100.