i teach preschool and my husband is in the army and we are in a huge military town. Kids no matter what age and even if there parents are military LOVE soldiers. they love the uniforms, the weapons, the tanks. Keep it SHORT and to the point. They have small attention spans. They will want to climb on you and touch anything you bring. If you are wearing your uniform, some kids might be scared (we had fire fighters visit last weeks, most loved them, few cried when he put on his gear)
i would think of one topic to say, keep it very short and have stuff to show them. the teachers will/ should be there to help keep them in order.