Sadie, my in-law's dog, was hit by a car Christmas eve night. We weren't told until just a few days ago, and we miss her. Sadie was growing increasingly deaf, blind, and absentminded: they think she just wandered away and never saw the car. To his credit, the driver did stop when he hit her and brought her to the house. He felt really bad, but I imagine a little black dog is an easy thing to miss in the dark. We don't blame him, and we're thankful he had the heart to stop.
I tried to explain to Aidan that "Sadie got hurt and the doctors couldn't fix her, so she died" but right now I think he still believes she's at the hospital and will get better. I don't think he understands...and I don't know that any 3yr old living a sheltered life really understands death.
But he misses her, and we do, too.