"He's doing pretty good right now, but I worry about, I worry about him as he realizes it's more permanent," he said, speaking of his son's reaction to Christina-Taylor's death.
Despite the pain of losing her, John Green said he wouldn't take back any of the time he had with his daughter.
She was born on 9/11, and was featured in a book about children born on that date. It was called "Faces of Hope."
She held that message close to her heart, and was proud to have been a 9/11 baby, her mother said. She loved wearing red, white and blue, loved the Fourth of July and thought of 9/11 as a special holiday.
"She was proud of that, she was proud of being one of the faces of hope, because she didn't really look at 9/11 as a tragedy like the rest of us did, she looked at it as an opportunity for change, for hope ...," Roxanna Green said.
"She was an amazing little girl and she got robbed of all the wonderful things that she could have done," her mother added. "Not only for her family and friends and her community, but for her country. We don't know what she would have gone on to do."
Her father has advice for others:
"I think the one thing I learned from this is keep your friends and family close because I don't think we could get through this without everybody's support, and I think maybe this fellow who did all the shooting, maybe if he had a little more of that, this wouldn't have happened."