Turns out they're pretty much always tuned in. But I still like to check sometimes. They've moved from noticing it, to being annoyed by it, to now making a theatrical production out of how annoyed they are by it (but are clearly enjoying it).
As a bonus, it keeps me on my toes for the time most entertaining to me to substitute:
I do not like green eggs and ham,
I do not like them, Gorilla-I-Am
Goodnight comb and goodnight brush
Goodnight nobody,
Goodnight brush,
And goodnight to the gorilla whispering "hush"...She even wants to swap the words that have plot-meaning -- like, why is the mouse wakeful, and how does a slumbering flea bite the wakeful mouse? /shrug. It's been my carrot to get her to try and read it herself.
If I change so much as a preposition when I'm reading a beloved book, my kids will instantly call me out on it.
I think as adults, we read to experience novelty and to change ourselves. Children are immersed in a world where they are surrounded by things that are new to them and often overwhelming. At bedtime, I think they want books for the exact opposite reason—for the safety and familiarity. To know that after all of the day's adventures, some of the things they already know—their memory of the book—are still true and correct. That they can build knowledge and it will continue to be relevant over time.
It's mind-numbingly boring for me, but they love it. I try to think of it like singing them a lullaby. For some reason, we're more accepting of repetition in song form over prose (how often have you listened to the same recording in your life?), but there's no real fundamental difference in them.
I don't know how many times he watched it, but it lost its charm. So it's not entirely about familiarity; I'm sure a big part of it is connection.
One has a banjo.
Spontaneity was really hard to re-run. =/