They'd be first cousins once removed. Removed means a generation older or younger. 

Second cousins are children of your parent's first cousins. 

[@Greenie] Nice post! Was a pleasure to read, I really like how Taria and Theon's relationship is developing. I just realized we're in the adorable family phase of the story like in the main series. We all know what happened after D:

Hmm, I'm gonna be interested in seeing what happens when these Greyjoy bastards face off with the Greyjoy bastard who is reaving all over the place.