There are only a couple clues in the text, and the rest just has to be inferred, based on what we know from their society. The first time that children are mentioned is when Montag and Clarisse are talking. She wonders why Montag and Mildred don't have any kids. Montag says that Mildred "never wanted any children at all," but doesn't explain why. Later in the story, Millie has all of her lady friends over to socialize and watch the t.v. walls, and they have a pretty revealing conversation about kids that seems to indicate that they don't want any because they are such a bother, an interruption to your life, and cumbersome. Mrs. Bowles, who has two children, first mentions that if you are to have them, it must be by C-section, because "no use going through all that agony for a baby." She views having kids as an obligation to humanity, to keep the human race alive. Then she mentions what a pain they are, but thank goodness that they are are in school 90% of the time, and the rest of the time you can "heave them into the parlor and turn the switch," meaning, stick them in front of the t.v. So, children are just a huge pain, but, they need to be had so that the human race doesn't die off.
Mrs. Phelps indicates that she doesn't want children because they "are ruinous." It's a rather ambiguous statement; she says this right after referring to how your hips are ruined, so maybe, children are bad for your figure. So, based on the clues that are given from these women, I would guess that they don't want kids because it ruins their figures and looks (they are very focused on being perfectly buffed and shined and fit), and because it is too much of pain. Children take up too much of your attention, are ungrateful, and keep these women from doing the things that they want to do.
Overall, their reasons for not wanting kids does not put them in a very flattering light. They seem to be shallow, self-centered, selfish women with no sense of love, decency or humanity about them whatsoever. It is just one more aspect of society that Montag, in the end, rebels against, and hopes to change. I hope that those thoughts helped; good luck!