Kids

That I recollect offhand, the subject is never mentioned or explained anywhere.

In some ways, though they are depicted as "friends" we could argue that Stephanie & Michael function as surrogate kids to Dick & Joanna. And when baby Steph is born, they dote on her like grandparents would (or at least like an older aunt and uncle would). So while there were not biological children for the Loudons, there was still a sense of family on this show. And I would even say Dick & George were like brothers after awhile.

Bob Newhart's character didn't have children on his 70s show either. In interviews he says he wanted to do a show just for adults. In true life though he has several children who would now be in their 40s and 50s.

Bob said in an interview that the reason he didn't feature children on his shows was because he didn't want to do the typical family sitcom where the children are the real stars and they constantly outsmart the dumb mom and dad.