I'm a Christian and if that book makes people want to know more about Jesus, I'm all for it. I don't think the child is lying, or they're trying to scam anyone.

That said...

I haven't read the book, but I saw the movie. The movie had a lot of feel good tropes that are nowhere to be found in the Bible - most notably, the idea that while Heaven is for real, Hell is not - and I left hoping that people weren't taking it as Biblical truth. However, I don't know how reflective the movie's mentality is of the book's stance on the afterlife. Does the book make it clear that those who have rejected Christ are not going to be experiencing that beautiful place his son describes?