I have enjoyed these stories since I was in elementary school, and now I am enjoying them again while learning Spanish. Overall the translation seems fine to me, but once in a while I notice something that appears to be a glaring error. I'll share two examples from El León, la Bruja, y el Ropero:
English: "There was nothing else in the room at all except a dead blue-bottle on the window-sill."
Spanish: "Allí no encontraron nada más, excepto una botella azul en la repisa de la ventana."
(A "blue-bottle" is a house fly, but the Spanish calls it a blue bottle.)
English: "The hags made a dart at him and shrieked with triumph...."
Spanish: "Las hechiceras le arrojaron un dardo y chillaron triunfantes...."
(Unless I am mistaken, the Spanish says they threw a dart at him.)
These occasional mistakes might not bother everyone, but they bothered me, and I thought they were worth mentioning. Again, the stories are still great.