Of course Green Acres was before All In The Family, and I was under 10 years old throughout the 60s, and furthermore I often did not understand the plots of each episode of Green Acres, but I recall that a pre-teen girl was cast as a regular character in the show at least during the later seasons.
I had heard that whenever a TV show has been on the air for a certain amount of time, new characters are brought in and when it is a child, it often means that the show has Jumped The Shark.

Sure, Green Acres aired before the term was invented, but I am wondering if the show's producers were trying to attract a younger audience, or try to find ways to make scenarios for funny episodes with a child as impetus.

I think that the little girl in Green Acres visited Eddie Albert and Za Za Gabor's characters and stayed.