Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
"I don't believe it!" could pretty much be our narrator's motto. He is amazed by everything that he hears and sees. This makes sense, especially for the readers, because we're talking about a world where stuff like witches turning little boys into mice is a regular occurrence. So, when our narrator finds out that witches have blue saliva, instead of just nodding along, he exclaims, "Blue! […] Not blue! Their spit can't be blue!" (3.77). The excitement and curiosity expressed by the narrator increases those sentiments for us as we read the book. If the narrator just had a "yeah, blue saliva... whatever" kind of reaction to everything, we probably wouldn't get into it as much. Also, if he say, "of course they have blue saliva," we'd feel a little distance from him. We like that his incredulous ("I don't believe it!") attitude is a lot like ours.
Along with this amazement is a sense of lightheartedness. This feeling seems to come more from the author than the narrator. Our narrator is actually scared quite a bit throughout the story. At one point, when he's stuck in the cook's pant leg, he even says "I heard shrieks of laughter coming from the other cooks but I can promise you I wasn't laughing myself" (18.37). We have a feeling that Roald Dahl, on the other hand, is laughing. How do we know? It's because we are, too.
When you find yourself chuckling as a witch violently proclaims, "This smelly brrrat, this filthy scum/ This horrid little louse/ Vill very very soon become/ A lovely little MOUSE!" (10.26), you hope it's not because you're a child-hater. More likely it's because Roald Dahl has a lighthearted attitude about it, so you do, too. This attitude makes it a lot easier and more enjoyable for us to read a book that's otherwise kind of dark and violent. If you don't believe us, go ahead and watch the movie version. The movie seems to be completely lacking in lightheartedness and, for that reason, it's creepier and more frightening than even the animatronics at Chuck E Cheese.