A STATE party to the convention against child soldiers may enlist someone under the age of 18. However under no circumstance may that person be used in hostilities.

"Article 1 - States Parties shall take all feasible measures to ensure that members of their armed forces who have not attained the age of 18 years do not take a direct part in hostilities."

Anyone, be they in a national military force or in an armed group not associated with a state, is a child soldier if he or she takes part in hostilities below the age of 18.

You could be in military service under the age of 18 and work the canteen, deliver messages, or help in the infirmary. For the purposes we're discussing here, that's not being a child soldier. But if you're under 18 and take part in any hostilities, you are now a child soldier. The military force you belong to has violated the treaty. If you're captured by the enemy, you're entitled to special treatment not afforded regular POW's.