The focus of the story is on the oppressive nature of the government, the censorship of knowledge, and the loss of human connection. The details of Mrs. Bowles' family life are not crucial to the plot, and therefore, they are not explored in depth.
The reader is left to assume that Mrs. Bowles' children were born through traditional means, but it's also possible that they were adopted or conceived through artificial insemination, as those practices were likely common in the society depicted.
Ultimately, the question of how Mrs. Bowles had two children is not as significant as the larger societal context in which she exists.