Two Charlies
I recently reread Charlie and the Chocolate Factory and Charlie and the Great Glass Elevator. It's fascinating that for two books that are so similar - same author, same series, same length, similar vocabulary levels and sentence structure - there are still several features of the latter that make it a noticeable, if small, step up in difficulty.
Most obviously, Chocolate Factory has 30 chapters while Elevator only has 20. As the books are the same length, chapters in Elevator are on average 50% longer. For books that may be amongst the first a child reads or is read to that aren't picture books, that makes a noticeable difference.
Elevator deals with more advanced subjects. At the end of the day, Chocolate Factory deals with very simple topics: exploring a chocolate factory, naughty children and a family that doesn't have enough food to eat. Sure, some more complex topics are mentioned, but they're essentially peripheral. In Elevator, the more complex topics are more central. Who is the President (and all these people around him) and why is he calling Russia, China and Mr Hilton? What are negative numbers?
The plot structure of Elevator is more complex. Whilst Chocolate Factory's plot is entirely linear and everything is as it appears, the first half of Elevator repeatedly shifts viewpoints between the Buckets and the Americans. What's more, a central element of the plot centres around mistaken belief: the Americans believe that the Buckets are first spies/saboteurs and then 'men from Mars', which of course they're not. Understanding this is really quite significant, given that 'Theory of Mind' is thought to develop between the ages of 3-5, with 'understanding others have false beliefs and act accordingly' coming relatively late on.
None of this is intended to be a criticism of either book - they're both fantastic. But the level of difference to me was surprising, as before thinking about it - and to an adult or even an older child - the level of difference is vanishingly small. But reading both to a child consecutively, one might find that whilst they're completely ready for Chocolate Factory, Elevator may be more stretching.