With this she's saying that due to their previous success, in school and work, they have all become wealthy enough to take it easy every now and again to play care free and time consuming game of golf. This is further emphasized in the next line where she states that "They drink their martinis dry" (Reynalds, Malvina. "Little Boxes"), which isn't typically a very expensive drink, but it's looked at to be a very posh or fancy beverage that is usually drank while you're trying to relax or have a good time. The rest of the third stanza talks about the children of these successful individuals, and how they too grow up, get educated and become just as wealthy as their parents. Although in the fifth line of the third stanza it states that "The children go to summer camp" (Reynalds, Malvina. "Little Boxes") which adds on another form education to help for later. This can viewed as Malvina saying that people may need some extra help or must be very determined and disciplined in order for them to become successful after university.
In the first line of the final stanza Reynolds says that "Boys go into business" (Reynalds, Malvina. "Little Boxes"). This is a use of historical imagery to show the common lifestyle of her time. This song was written in 1961, when men were still typically the only ones to go college and become "doctors and lawyers and business executives" (Reynalds, Malvina. "Little Boxes") while women were usually house wives and raised children. The listener of this song can also interpret that the song is saying that everyone lives life repeating the same cycle and that everyone is basically the same. Throughout the first stanza of the song, the terms "Little boxes" (Reynalds, Malvina. "Little Boxes") is used in repetition to refer to the individual lives of people. It describes that there are multiple "boxes" that are colored differently or appear to be different to the naked eye.