It really is common for kids this age, particularly boys, to regress. During the daytime they play so hard that they're not willing to stop for a pee break, at night, they sleep so hard that they sleep right through it. Talk to your peds about it, they'll give you some literature. It could even be a bladder infection (more likely for a girl than a boy in that department.)

So sorry to hear about grandpa, there are some really good childrens books out there to help them understand these things.