Omgggg okay I imagine this starting with Sam and Cas talking about maybe hypothetically having children and they’re both down for it but it would be super difficult to arrange, and then Charlie show up like “What up, bitches, I’m in some kind of trouble and need protection, so one of you needs to knock me up or something,” and she says it TOTALLY JOKINGLY, because hello, lesbian, but then Cas sits her down (and Sam hovers awkwardly behind him) and proposes that she be the surrogate mother of their child. Sam could be the father, and Castiel would donate some of his Grace for both the baby and Charlie’s protection. And Charlie is totally down for that because 1. Even if she was joking, the pregnancy thing was kind of her idea, and she wouldn’t joke about something like that lightly, and 2. Holy SHIT Sam and Cas are so cute together and they would have the cutest baby hell yeah I want in on that.
So Charlie uses her forging/hacking powers to get herself and Sam into a clinic to do the thing, and I’m not 100% sure how that works but it happens and she gets pregnant, and as Cas promised, he gives her some of his Grace to protect Charlie and the baby (but it’ll leave when the baby’s born because that’s what the Grace is attached to).
And they dote on Charlie hand and foot when she lets them (she thinks the pampering is nice at first but gets old fast, and dammit, she’s an independent woman, she can get her own ice cream thank you). Sam feels pretty awkward about the whole thing throughout the entire pregnancy, but he holds Charlie’s hand during labor (because they all think she and Sam are together probably and it’s the least Sam can do for her) and is absolutely in awe when he sees his daughter for the first time.
Charlie says something like “Hell yeah, it’s a girl. I’m gonna spoil the shit out of her. I’m gonna be her favourite aunt.” And that’s a little strange to Sam because she’s technically the baby’s mother, but he’s not thinking about that too much because holy shit he has a daughter and she’s so tiny and as soon as she stops crying and someone hands him the baby, wrapped up in blankets like a little pink burrito, he sees she doesn’t look really anything like Charlie. She has a shock of dark hair and tilted eyes and pink lips and really, they’ll have to wait and see what happens when she grows up, but Sam would be willing to bet she’ll look just like him. That gives him a swell of pride stronger than he ever could have anticipated.
So Charlie rests up, and the moment they leave the hospital, she hands the baby over to Sam and Cas, stays in the bunker a few days to discuss visitation and stuff like that (”dudes, I’m in and out anyway, I’ll just pop in like I do and see the little tyke”), and then she’s out, and Sam and Cas are left with the little bundle of joy (who’s not so very joyful when she’s screaming at four in the morning but it’s all a blessing anyway).