I don't see a problem with this. I think it's great that they have it in a book. Why? Because most parents will not talk about these things with their children or just don't know how to broach the subject on a child's level. I can remember when I was a little girl I needed to have my tonsils out. Anything my parents told me was frightening. My surgeon didn't sit me down with my parents to talk to me. But my parents bought me a book about a little boy my age who was having his tonsils out. I read it a hundred times before my surgery and it made me feel so much better about what was going to happen.
It's a lot more reassuring to a child to see a familiar character, like Olivia, going through something they may have to in the near future than to simply have a parent tell them all about it.