Trivia: Among the girls at the slumber party are Florence Henderson's daughter Barbara ("Ruthie") and Robert Reed's daughter Carolyn ("Karen"). They interact briefly; during the game of Truth Or Dare, Ruthie is dared to go check on the boys and asks Karen to go with her. In addition, Hope Sherwood ("Jenny") is the daughter of executive producer Sherwood Schwartz.
Trivia: In Season 2, out of the five episodes with one of the kids not appearing in the entire episode, three episodes used the name of the missing kid in the dialog most likely to make their absence from the episode more unnoticed. (Marcia/Peter/Bobby in these cases - each one's name mentioned at least once even though they never appeared). The episodes were ("What Goes Up...", "The Tattletale" and "Coming-Out Party"). Also the 4th episode of season 2 - "A Fistful of Reasons" contained Jan being mentioned yet she did not appear in the entire episode.
Answer: Much trivia has been written about "The Brady Bunch, " including the various interrelationships and dynamics among the members of the cast. For example, much has been written about why Robert Reed's Mike Brady did not appear in a couple of episodes, including the series finale, due to rancorous creative and artistic differences with series creator, Sherwood Schwartz. However, in contrast, I have never run across any reasons given why a particular child did not appear in specific episodes - only that the episodes were missed. This suggests the causes were likely unexceptional, such as illness, injuries, vacations, or real-life family obligations.
Michael Albert
Although those options ARE possible reasons, it just seems a bit ironic that this happened all in Season 2 ONLY and within a short amount of nearly consecutive episodes. Never happened during the other 5 years of the show at all.