As he is a writer of fiction and, more specifically of children's fantasy fiction, Roald Dahl's writing style is a mix of Plain and Sweet, according to Walker Gibson's Style Machine.
Most of Dahl's narration is Plain:
-high frequency words
-1st person pronouns
-action verbs, active tense
-compound sentences (lots of coordinating conjunctions “and”)
Occassionally, Dahl moves to Sweet (like Dr. Seuss), especially when his Ooompa Loompas are singing:
-2nd person pronouns
-2nd Person (You-oriented)
-little reasoning needed
Here's a sample from "My Grandmother":
"I myself had two separate encounters with witches before I was eight years old. From the first I escaped unharmed, but in the second occasion I was not so lucky. Things happened to me that will probably make you scream when you read about them. That can't be helped. The truth must be told. The fact that I am still here and able to speak to you (however peculiar I may look) is due entirely to my wonderful grandmother."
Notice the Plain Style: the first person pronouns, intimate voice, simple sentences, contractions, monosyllabic words, high frequency words, lots of articles (Plain style is the language of fiction, confession)
Notice the Sweet Style: use of parentheses, 2nd person pronouns; the last two or three sentences read like a movie promo (Sweet Style is the language of advertising)