Yeah...just read a different book. If they aren't ready, they are not ready. Either wait until they are older or give the info they need to read the book with an appropriate amount of maturity. No need to put in so much extra effort and ruin a series your son might actually end up enjoying as it is.
Sidenote: I wonder if her child has discovered the internet yet. It just might be a rude awakening...
Sidenote: I wonder if her child has discovered the internet yet. It just might be a rude awakening...