Right when my oldest son brought the DVD home from work, I remembered watching it when I was younger. He said he showed it to his World History class (the whole class was 15+). I had to watch it first, but when I did I decided it would be a good idea for my grand kids to see. Both of them lost their parents in the Army, so they had a little more insight than some kids do already. I wouldn't recommend it for all children, but for ages 13+ and any other kids who are mature for their age should be fine. Its not near as bloody as a lot of the video games or movies a lot of kids see.
The 3rd recipent of the honorable Best Picture prize, "Western Front" is easily one of the greatest movies I have ever seen, and not just about war! It shows you the graphic, painful truth, and how very young people are going into the Army and dying within their first few days; they enlisted because they thought it would be "fun." Not just for movie buffs, but for every(mature) teenager and up!
Quite violent, but that's the point. It shaped my views of war a lot. Basically, the point of the movie is that war is hell. You should definitely watch it. I saw it when I was twelve.