I do think Rebecca wanted/wants children. When Rupert/Giles shows up at her office in "All Apologies" (Season 1.9), he takes delight in taunting her. It's obvious in Rebecca's expressions that his words hurt. After this horrible exchange, she rushes to Ted's office and confesses.
"Rupert: People change. I do want a child. I just suppose I... I didn't want one with... Before. I mean, in the end, it's just about being with the right person, isn't it? Oh, I'm so glad I got to tell you this in person. I would have hated for you to read it in the press. Good luck against Man City."