B.G.
Oh, man, B., this is a tough topic.
As far as comfort goes, I can only say I've survived it with three kids and an intact marriage. My spouse went from Navy, to security guard, to county cop, to city cop and is now looking into federal positions after obtaining his BA. I know a lot of other military/police wives, and it takes a delicate balance and a lot of hard work from both sides.
Our middle daughter is ADHD with sensory issues, and has a terrible time dealing with change and departure. I've found my consistency and reactions are more influential than the amount of time dad's gone, or how scary the news is (another parent at Kindy asked me in front of my kids about the death of one of my husband's fellow officers). During Navy deployments (and the police academy), we'd fill a jar with jelly beans for every day daddy was supposed to be gone, and take one out every day, so the kids had a visual reference. We also wrote a lot of letters, had a consistent routine, and answered the same questions over and over and over again.
It sounds harsh, but don't be afraid to consult the professionals. A family counselor can mediate before the decision is made, so that "every thing's on the table", as well as provide support and resources for you in case things get crazy while he's gone. Web resources for military families as well as the police officer's guild are good reading for suggestions and support groups.
Good luck, and know that if it's truly his passion, he won't likely be happy in any other field.