My son just finished reading Pirates Past Noon last night, and he was telling me about the ending (there be spoilers here, but I doubt we have a lot of 2nd graders reading the Wordhoard).
Apparently, at the end of the book, it turns out that the children were receiving anonymous help from "The Mysterious M," who as it turns out, was Morgan Le Fay disguised as a talking parrot. As part of the broader feminist attempt to rehabilitate Morgan, she's depicted as a "good guy" in this book; when one of the children says that she is a witch, she corrects them by saying she is an "enchantress," and that they should not believe everything they read.
So, what is Morgan doing helping children against pirates in a magic treehouse? In addition to being Arthur's sister (no mention is made of potential incest), she is also the Camelot librarian, and is traveling through time to collect books for the Camelot library.
This struck me as a rather abrupt eruption* of Arthuriana into a book about pirates. I think I'll encourage my son to see if his library has The Knight at Dawn from the same series, to see if it too is Arthuriana.
*Abrupt eruption = "abruption," a word I just made up. Now you owe me a nickel each time you use it.