D.J.
I agree with one of the posts that you should decide on how to tell each child based on their own personality. One may do better with sooner and the other with less or no notice. And for children this young, a couple days is usually long enough considering their concept of time.
And I just wanted to reiterate another post that noted how you both approach it will be the biggest determining factor on how the kids handle it. Children can read the emotions of adults even when they use no words. Use this time to get yourself ready so that when it is close to the time to go, you are comfortable with it. If you are not, nothing you can do will make your kids comfortable with it.
A few suggestions that might make this easier:
1) if you have enough time before the trip, have your son stay overnight somewhere for one night away from you both. He'll have the experience but more importantly, so will you.
2) make a video tape (vcr/CD-Rom/DVD) of you and your husband talking to the kids, holding up some of the toys they can bring, talking in an excited voice about all the fun things they will be doing with the grandparents that your in-laws can play for the kids while you are gone. Maybe 5-8 minutes long. Don't focus on "I miss you, etc etc" but keep it upbeat, happy, excited and fun. You can tell them you love them and you are excited to see them soon. Just avoid anything sad or emotional that will tug at their hearts to be sad as well.
3) if you are comfortable with giving a special gift/toy in this situation, giving your in-laws a special gift they can give each child (from them not you), that might be one extra special touch and memory for your children they can take back with them when they come home.
These times away are so important for both the child's growth and independence and ours as parents learning to slowly separate over time.
Good luck and enjoy the trip with your high school sweetheart.
D.