Perro grande...Perro pequeño - Easy Spanish reader by P.D. Eastman. Recommended in Lesson 16 - Grande y Chiquito of the Sonrisas Curriculum Level I (under Spanish Elementary Curriculum).
Two dogs are opposite in every way, but are the very best of friends. Unlike a lot of other books that are translated into Spanish, this one doesn't lose much of the rhythm or the fun when translated. It also has the English text along with the Spanish text. Paperback, 32 pages.