My cousin's dead, are his children hallucinating?

My cousin's wife cheated on him and took the kids to go live in another state. A few days later he got into a car accident and died. The kids don't even know about this...didn't even come to his funeral. They're both 7 and 3...that night when their dad died, they had a dream that he came and said, "Bye bye." Then they keep saying things like, "Why is daddy always behind you? How come daddy doesn't talk he only smile?"