It all depends on the gene formation in the brown eyed parents - if they had one blue eyed and one brown eyed parent they will have both genes there to be picked up the next generation down (I think it becomes a one in four chance) but if both their parents were brown eyed the odds are slashed drastically. So, whilst possible it is much more likely for them to produce a brown eyed baby than one with a different colour eyes.
However, now i've been thinking about this, i'm pretty sure Vic Windsor had blue eyes so whilst Viv's dominant brown obviously came through for Donna, the blue gene will be there in her DNA and must have manifested with April. And of course we've never seen Marlon's parents though the Dingles tend to be dark eyed. Charity is the only blue eyed one I can think of.