This is a portion of the artist Robin Jacques' frontispiece from A Book of Devils and Demons by Ruth Manning-Sanders.
I love, love, love that they're just sitting there with a book!
I'm not sure what these creatures are supposed to be. The goat-like legs and pointed ears suggest they are fauns. But the wings on the adult creature have me a bit confused. Perhaps, as the book title itself states, they are supposed to be some sort of fantastic beasts with a demonic or devilish bent. (And, because this is a children's book, the author equates most devils and demons with "tricksters" who are just out to "pester" humankind. Nothing too blatantly evil or horrific.)
And how bad can they be, if they like books?