I'm going to have to say child soldiers are the most frightening in combat, 8-12 year olds being the worst.

These kids are usually drugged up, forced into whatever war their adults place them in, many have been sexually abused. No grown-up soldiers wants to kill children, even if they are trying to kill you. Netflix had an incredible movie called Beasts of No Nations, it was about how a young orphaned boy ends up as a child soldier.

Most of the enemies of the free world haved used child soldiers in one capacity or another. A prime example is the unarmed child waves the Iranian military used against the Iraqis during that war. The Iranians used 10-13 year old boys to run forward of the main body to clear mines and other obstacles as well as being ‘human shields' for the Iranian military during attacks.

The Palestinians have used toddlers straped with explosives and set them against Israeli Soldiers. This happened 100s of times during the Vietnam War. The VC would strap grenades to small children and send them towards the American lines. I would say child soldiers are the most frightening in combat, because even the most combat hardened soldier will end up hesitant to shoot a child soldier.