EmmyG---on your question on what to tell the boys---remembering my own children--one tip that I would give is to ask the kids what they would like to know. This will give your first clue as to where to start--and, address their real concerns---rather than what we assume they want to know.
For the 2yr. old--I would put out a couple of pictures of his dad--where he can see them during the day. That way he would know that he didn't just evaporate.
dandylion