YA (or children's?) book about a girl trapped in a fairytale?

I read a book at a friend's house at a sleepover when I was younger- probably like 10-15 years ago? I don't remember much about it but it followed a girl that somehow ends up in a fairytale.

At some point she made a deal with Rumplestiltskin in exchange for her firstborn child and so she (somehow- maybe her fairy godmother did it for her or something like that?) goes to the future and brings back her firstborn child. I remember the scene where she is with the love interest of the story in a carriage or something and she mentions that the baby has a crooked pinky I think it was and the LI says something along the lines of "it's not that rare, its genetic, look I have it too" and I think it was kind of a realization moment that this was their future kid together.

My memory of literally every aspect of the story beyond this one specific scene is lost so I may not have the premise totally correct...

This has been on my mind and torturing me for weeks, please tell me someone knows what book this was!