When dealing with real risks that involve the possibility of death/serious injury, we are very honest with DD (there seems to be little point in obfuscation or euphemism with her).
There are some risks that are real and that we must simply learn to live with. Emotionally, our kids may not be ready for the awareness that advanced cognitive understanding gives them. Once they know, they also seem VERY sensitive to anything that seems to brush off their very valid concerns, too. DD can smell patronizing efforts or dismissal a mile off. SO first things first, acknowledge that his perception isn't "wrong." But then gently suggest that there may be more to it than he's thinking... that there are nuances that are important.
Here is how we manage the feelings of stress:
1. Emphasize the measures that are/will be taken to mediate risk (explain how body armor will help keep Daddy safer, and how he will wear it all the time, how good his training is, etc.)
2. Point to examples of how others have survived just fine-- preferably people that your family knows,
3. find positives in the risk-- that is, while we can't always control our circumstances, if we can bear in mind the hidden 'gifts' that they give us, or look at the big picture and recognize the greater good being done (Daddy is going to help people in Afghanistan, isn't that wonderful?) it can help,
4. Data. If one looks at the total numbers of deployed troops in the region over the past decade, the rates of injury/fatality are actually quite low. This risk isn't as high as he's probably envisioning, and while none of us want a family member in harm's way, the statistics suggest that it is PROBABLE that he'll be fine, other than all of you missing one another terribly, and finally--
5. We're family, and family sticks together by sharing their troubles. Even mommies and daddies worry sometimes about things that we can't really change, but as long as we talk to the other people in our family about our worries, we can sometimes find ways to make stressful times better by facing them together.
All of my best to your family as your husband deploys. {hugs}