Blue eyes are recessive. Meaning that you must have two of the blue alleles in order to have blue eyes.
Brown is dominant, so as long as you have at least one: the blue allele will not show.
In order for two brown eyed parents to have blue-eyed children is if both of them were heterozygous, or they both had one brown and one blue allele.
If you work it out on a Punnett square, that's the only way for them to have blue-eyed children.
Hope that helped!
