Story reading time

Trying to be a good dad I read to Anthony and Ernest when I'm not traveling. We did the Narnia Chronicles, The Time Apprentice, The series of Unfortunate Events, Neil Gaiman's Stardust (which was a challenge, since there are quite some adult passages in the book) and some smaller titles. Since daddy had to buy Harry Potter #7 I suggested to read all the volumes to them. When we pick a book to read, I usually pick 2 or three and let Anthony and Ernest choose which one they like better. This time I offered Harry Potter #1 and The Celestine Prophecy. After flipping a few pages Anthony insisted to read the prophecy which made me smile, since the book was part of the reading and discussion material for my counsellor training. It means quite a bit to me.
Actually I was quite sceptical if they boys would enjoy the story. However they could recall very well what happened and insisted to continue despite my offer to switch to Harry. So the experiment goes on. I'm curious how much of the book will be comprehended by two 7 year old minds.