The Garretts are two tall people, played by Jordan Baker and Kevin Kilner. They sport large noses. A number of nose-related jokes ensue. I don't really get jokes related to physical appearance although in this case, they are related to the idea that a baby might inherit any number of characteristics from its parents and grandparents.
They are a wonderful couple (and they come through on their promises to Roz: later, Roz references Alice staying with her local grandparents). They are sweet-tempered, kindly, friendly, down-to-earth. They neither castigate Roz nor try to take her over. They like dogs (yes, two big schnauzers). They enjoy quiche. They are naive but not unsophisticated (they are heading to Paris).
Fantastic couple who fall into the category of "actually, opposites don't attract--similarities do."