I have to admit, I am really surprised by the reactions of my niece and nephew to this book. I really thought that they wouldn't get it, and therefore wouldn't like it. I was totally wrong on both counts.
First of all, the kids got this book, and the reasons for Peter's loneliness and sadness. Although my niece and nephew were not abused as it is implied that Peter is, like Peter they are children of divorced parents. They related to Peter and his family situation, and I think that they understood Peter's need to escape and decompress, his need to be somewhere he considered safe and perfect, much better than I did. They were both completely pulled into this story--mesmerized by it and lost in it.
The fantastic (and fantastical) illustrations also pulled the kids in, making them explore the wonders of Peter's garden right along with him. The kids also seemed pleased with the ending of the book, my nephew even stated, "that's good. Now Peter can go in his garden whenever he wants to." Both kids were enthusiastic in their praise of this story.
For me the magic of this story was reflected in my niece and nephew. The story was complex, full of beauty and ugliness, and it was happy and heartbroken all at the same time. This story showed me my niece and nephew, and showed me their complexity, too. Kids are deep little things, and this story reminded me of that.