How do you talk with or counter adverse feelings of a child about getting rid of a pet?

My daughter was given the dog for her 18th birthday. Two months later,she was in a fatal car accident. We are in the process of moving and some of our housing options don't allow pets. My six year old does not want to get rid of the dog. He gets sad when we discuss getting rid of the dog. However, we would be giving the dog to my aunt and he could visit her on occasion.