Hi! I'm desperately trying to find this story: Dean and Cas had a kid, Liam. However 'cause time in Heaven is way different than time on Earth, Dean saw the kid only a few times in his life. He lied to his relatives saying that his love and son were living in New Zealand.
In the end, Dean died of cancer and Cas took him to Heaven with his dog, to live in the nest the angel built for Liam when he was a baby. Then they had another kid, this time a girl.
Sam was very angry when Dean died but Liam, who was with his human father during his last hours, finally gave his uncle a piece of his mind. Sam should have been happy that Dean was now in Heaven with his mate; he was just behaving like a spoiled brat because he was finally alone like he wanted all his life.
Sam's daughter was the only one who understood what Liam really was: an angel. However a very special angel because he never forgot his parents, like instead all the other fledglings did.
I remember that, when Liam was a small child, Cas left him with Dean for an emergency with the Host. During a storm, Liam was very scared but Dean assured him and drew with him.
Cas was punished for his sins during the Apocalypse with having to work as a Heavenly Librarian???  And Liam got in Gabriel's Host.