Changing standard storytelling as an exercise in challenging kids. Plus, it’s fun. I’ve done these and similar things with my nieces. The fill-in-the-last-word is something I learned from someone with a background in child development, and the replace-a-word I do just because I love kids’ sense of the absurd (and finding the line where pretending becomes just silly). Changing the story has its dangers, though — it can be upsetting if you aren’t doing it right and the child isn’t in the mood for it.
Uncle’s tip: Always negotiate how many time you’re expected to read the book before you start. They’re minors and contracts aren’t legally binding, but it’s some leverage, at least. You get tired of reading the book long before they tire of hearing it … again.