It's just classic Chekhov's Gun; come on, basically all stories follow this advise. Unless the author simply brought it up to explain why they don't have children, in case a Japanese readers might feel it strange that a woman in her 30's doesn't have kids, there's no reason to mention them.
But I don't buy that's enough of a reason, so she'll probably accidentally get pregnant; or at least it'll be a major explanation why she got married in the first place, or why she can't be with a woman, or whatever.
I'd be really surprised if this was the only time in the plot kids came up after making such a big deal out of it (talking with the mother-in-law, talking with the husband; and of course also the ex with her kid...)