To come up with a thesis, try to think about what the relationship is between Harriet and her malevolent fifth child, Ben. What do you think Ben represents in the story?
One possible vein to explore is whether Ben represents the suffering and hostility that Harriet may harbor about her weary life bearing and taking care of children. Lessing describes Harriet as "pale and strained because of morning sickness . . . because she had spent a week scrubbing floors and washing windows." Harriet wears herself to the bone giving birth, cleaning, and taking care of relatives, but she never complains because she has committed herself so fully to this kind of life. Is Ben a symbol of her resentment—a feeling that she has long harbored but has tried to hide?
Ben might also be a symbol of Harriet's resentment about the way other people regard her and David, her husband. Even Harriet's mother criticizes her for having so many children so quickly. Lessing writes of Harriet and David, "They were wondering why it was that people always criticized them." Is Harriet, in the end, unable to show compassion toward her first four children, who suffer because of Ben, or even toward Ben because she has long received no compassion from others?
Another idea is that Ben is an outgrowth of David and Harriet's selfishness. While they believe that they are the antithesis of the times, the "greedy and selfish sixties," they are also considered selfish. For example, they imagine Harriet's mother, Dorothy, saying, "You are very selfish, both of you. You are irresponsible." Perhaps in their effort to be so giving and selfless, they are selfish in that they think only of themselves and their dreams of having so many children. They are oblivious to the way in which their children affect Harriet's mother, for example, and Ben is a potential symbol of this greed and selfishness. There are many directions you could take in this paper.