Katherine:
I realized each family's situations are different so take some of this 'for what it's worth'
If you do have to be away from the boys for the whole trip you can do some little things for them for each day you're gone. Leaving notes for them, gifts for each day, notations (big visual cues) on calendars, to make it easier for them as well as you.
Our daughter was 3 when we traveled and also had limited expressive speech at that time. We did all of the above which helped her to have things to do, to remember/ know mommy and daddy were coming back, and to have things to look forward to each day she was at home with her grandparents. It also helped me cause I could ask her each day about the gifts she'd gotten!
joanne