We see Bud with that Skippy character and in the later seasons the blonde with the buzz cut, but I do not remember the girls hanging out with the same person over and over again.

Did I not see them?

Yeah, the blond kid is Claude, and in the earlier seasons there is Joe Phillips.

With Kathy there is Patty Davis next door--she is mentioned in multiple seasons but isn't on much. She is a different actress in the final season than in earlier seasons.

With Betty there is no one consistent friend. One I've heard a few times is Dotty Snow, who was introduced on a quiz show with Bud, as being from another town. A few episodes later, maybe in the next season, she is suddenly Betty's BFF. Then she kind of disappeared--maybe she moved back to the other town.

Yvonne Lime played her in 12 episodes (per Wiki). Yvonne married producer Don Fedderson (My Three Sons) in 1969 and is now 85. In her role for FKB she was kind of ditzy, the way Betty acted in the first season.

Just did some poking around about Patty Davis, who early on was played by Tina Thompson, and in season 6 by Reba Waters (who had played another little girl in an earlier episode, Kathy, the Indian Giver).