How is it possible for two parents who both express the dominate phenotype to have offspring with the recessive phenotype?

A phenotype is what you look like. The parents are showing the dominant phenotype. If we were talking about eye color, the dominant trait is brown eyes. However, the parents might have a recessive gene which is covered by the dominant one. Say they both have Bb genotypes. The capital B is brown and the lower case is blue. Their blue gene is still there, just covered by a brown one. In a punnett square, there is a 25 percent chance a child will have blue eyes (bb) In most cases, the children will end up will brown eyes. Hope that helps!