Sure, younger kids may think the real point of Hallowe'en in the candy or the costumes. But they're likely to notice some of the scarier motifs that pop up in the decorations, and this presents as unexpected opportunity for some learning.
A Drop of Blood by Paul Showers, illustrated by Edward Miller.
The text of this book is straight-ahead science for the grade school set, explaining the key components of blood (red blood cells, white blood cells, platelets) and what they do. There are nice diagrams of how the circulatory system gets involved in transporting nutrients as well as oxygen, pictures of a white blood cell eating a germ, and a step-by-step explanation of how a scab forms.
But this unassuming text is illustrated in classic horror movie style.
All the "people" in the drawings are either vampires or ... uh, whatever those greenish hunchbacked creatures who become henchmen are. And this illustration choice is brilliant! Kids who might be squicked out by blood in real life cannot resist the scary/funny/cool cartoonish vamps accompanying the text in this book. The drawing of the Count offering Igor a Band-aid for his boo-boo is heart-warming. So is the multigenerational picture that accompanies this text:
Little people do not need much blood. Cathy is one year old. She weighs twenty-four pounds. She has about one and a half pints of blood in her body. That is less than one quart.
Big people need more blood. Russell is eleven years old. He weighs eighty-eight pounds. He has about five and a half pints of blood in his body. That is a little less than three quarts.
Russell is a young vampire, while Cathy is a cute green toddler with purple circles under her eyes.
This is a really engaging book. And, the science looks pretty good.