Dear B.,
When my brother and I were the same age as your boys my father was doing a Naval Exchange tour of duty. He was an Air Force fighter pilot training to fly with the Navy. His squadron was sent to do a tour of duty on the USS Enterprise for almost a year. We sent care packages to him and letters. We also used a cassete recorder to record a mock radio broadcast (this was about 1972). The tape would have news from home, stories about what was happening at school, we recorded his favorite musicians, etc... Dad would rerecord over the tape and send it back to us. He read stories, sang our favorite songs, and told us about the places he was visiting. Every month we both recieved a postcard from him with a short note. My mother would glue the postcard into a scrap books for us.
With today's technology you can do all sorts of things besides letters. My husband bought two web cams so that when he travels he can take one and we have our own family web confrence while he is gone. Using a video or DVD camera to record messages and events can be something your boys might enjoy. You can also pick out a calendar to mark off each day until he is due to get back so they see time passing.
As for explaining his leaving I would tell them the truth. He is a Marine this is part who he is as a person. Being a soldier is tough, sometimes they have to leave the people they love for a while, but that doesn't mean he is gone forever. Hope this helps.
J.