So, okay. If Jon and Dany do have a child, it's not bittersweet for one of them to survive and the other die, to leave the survivor to raise the child. It's just sad. I was raised by a single parent after the other died during childbirth to a sibling. It's been over 2 decades since and it still eats my father up that my mother is no longer here, Even though he got to raise 3 kids. I say this through experience of talking to my father(after we've both had a few drinks) that it's not bittersweet that he has us kids, rather it's just sad seeing him talk about my mother as if it all happened yesterday. It's something I can't really explain unless you experience it.
It's super selfish of me to say that because others have a different interpretation of everything and I'm sure it can be written in a way that it works as bittersweet, but I've lived through a similar situation as people have described that they think will happen and in my experience it's not bittersweet.
Quite honestly I could see a more bittersweet moment being if all of the starks die except Jon and then Dany and him raise a child as the next heir, which ends up being the last descendant of two great houses.