What might cause a two year old child to throw up and run a temperature 6 hours after lunch?

Many, many things: It could be something he ate. It could be a virus he caught from someone. It could be some other kind of infection that happens to be causing fever and vomiting. The bottom line in a child with fever and vomiting is that he need to stay hydrated. Give very small sips of liquid (1-2 teaspoons) frequently (every 5-10 minutes). This way, even if he vomits, some of that liquid will be absorbed.