I don't get why you don't understand using the show's PBs. Pretty much every book fandom uses the adaptations actors as their PBs once they come out.

As for the age thing, it's definitely the uncomfortable playing such young characters in such adult situations.

I don't think it's be considered AU if you pull from both the book and the show. Wasn't it partially George's idea to age them up for the show? And the younger characters pretty much stay the same between book and show. I didn't notice any differences between anyone besides age. And Robb being a douchebag.