Most people have heard that the genes for brown eyes are dominant. So how is possible for the two brown-eyed parents to have a blue-eyed child?
The reason is that genes come in pairs along the twisted double DNA strands. If both of these genes code for brown eyes, you will for sure have brown eyes. But you can also have brown eyes if one of the genes codes for brown but the other gene of the pair codes for blue -that is what dominance means. You will only have blue eyes if both of these genes code for blue.
So, your grandchild has both of his two genes coding for blue eyes. He got one of those from the mother and one from the father. So it seems both of his parents have a mixed brown/blue pair of eye color genes. But both of them have brown eyes because brown dominates the blue gene. Getting one gene from both parents for his pair of eye color genes means he had a 25% chance (one out of four possible combinations) to get those blue eyes.