Thursday's Child question

When I first read through Thursday's Child by Sonya Hartnett, I found it to be strange because it does not seem plausible that a parent would allow their young child to dig tunnels underground, let alone live in those tunnels. But as I continued reading, I began to believe that Tin doesn't physically exist throughout this text and is instead a creation of Harper's mind. After studying this some more, I believe that Tin actually did die in the mudslide at the beginning of the text. Instead of physical existence, he exists within Harper's psyche alone. After his death, Harper, because she feels responsible for his death, sees Tin as still being alive and furthermore, sees him as an extension of herself. In addition to this, I believe the Flute family as a whole plays along with Harper's portrayal of Tin being alive because it is easier than accepting the hardship for what it is. Having Tin alive allows them to blame him for their difficult life. I have more that I could say regarding the reasons I believe Tin to be a part of her psyche and not physically alive, but what do you think?