I don't have perfect recollection of every Charmed episode. Because of that, there will most likely be a bunch of inconsistencies between this and what actually happened in the show.
Prue had forgotten about Peyton when she decided to tell Andy the truth. She just had to know what he thought about magic. But then the conversation turned to their future.
"So when you have kids-"
"If they're girls, yes."
"Is it something you can get rid of?"
"No. Can you accept that?"
"I don't know."
Prue's heart sank. That's when she remembered Peyton. If he couldn't accept the idea of having a daughter for a witch, what about her? Would he pretend that she never existed and go on living life?
"If you can't accept me being a witch, then fine. But what about Peyton?"
Andy's face went pale. "Peyton?"
"Yes Andy, Peyton. You know, our daughter...the one you keep a picture of in your desk drawer. The one with your eyes and your chin. The little girl that you didn't want to give up in the first place."
"What about her?"
"You just asked about when I have kids. Well, Peyton is our daughter, Andy."
"And that means-"
"Exactly. I'm a witch and Peyton's a witch. Can you accept that? Would you really reject her for something she has no control over?"
Andy let out a heavy sigh. "We gave her up for a reason, Prue. And she has a good life with the parents who adopted her... Do they know about-"
"Magic? Yes. They know she's a witch."
Andy flinched. "Peyton's a witch...my daughter's a witch..."
"Yes Andy, our daughter is a witch. Can you accept that?"
"The parents she lives with understand what she is. We've never met. I just don't think..." A few seconds passed in silence. "I don't know what to think, Prue."
The truth spell was almost over, Prue had to have an answer. Not only for her sake, but also for Peyton. But the answer hurt more than she could ever imagine. "It's just not a future I envision happening," Andy said.
"And Peyton?"
"I'll always love her, but-"
And that was it, time ran out and Prue didn't get an answer.
Again, sorry the dialogue and situation doesn't exactly line up with the episode.