I work at a children's organization for at-risk preteens, and on Veteran's Day every single kid wrote a moving if not always coherent letter to a few soldiers serving in Afghanistan who trained with one of my coworkers. Some of them were honest ("I'm not sure what you do, but thanks"), some were confused ("it must be awesome riding on private jets") and some were inevitable ("I hope you killed a lot of terrorists") -- but all of them were sincere in the special way that only kids can really nail down. Sincerity is important. This is a good article.
I hope the people that get them understand they're just kids and everything. On second thought, "I hope you killed a lot of terrorists" could either trigger trauma, or make a guy's day for being this kid's hero. So it's not all bad :D