Sounds like the subtle approach may not work with her, since she came on so soon and so strongly from the get-go, and has continued to push the issue.
Reminds me of friends of ours who had decided to delay having children for a few years. He happened, though, to be a pastor, and as he stood at the back of the church to greet the exiting parishioners, he'd be met by the dear lol (little old ladies) who'd ask, as they shook his hand, "when are you going to have children?"
He took to answering each inquiry with, "When the sperm count comes up." They didn't ask much after that.
If your dad wants to date this woman, I suppose it's his business. But her comments to you are YOUR business, and not being right there and getting a "feel" for what approach might work in this situation, I really don't know what to advise. As this is a private decision between you and your spouse, it is really none of her business. Ann Landers would probably say "MYOB" to her, lol! Formulate a pat vague answer (That is a private decision, and I really don't wish to discuss it with you, or whatever you come up with that you are comfortable with.) and repeat it every time she asks. She may get the message someday.