The first few years of childhood seem marked by contrasts: babies are unequivocally small, adults are unquestionably large, and the area in between goes unexplored. But a little later in child development, those stark oppositions give way to more subtle comparisons. “Pomelo’s Opposites” and “Is It Big or Is It Little?,” two stylish new books for the preschool set, play with ideas of difference in ways that are sure to provoke thought and giggles.