Militaires Sans Frontières

Child Soldiers without boarders.
Spencer, Maj. R.V. “Korean War – HD-SN-99-03144.” Photograph. U.S. Army Korea. 

The very first time the United Nations conducted a global study of the prevalence of child soldiering was in 1996. It was a UN study titled Impact of Armed Conflict on Children, and stated that there were approximately a quarter of a million child soldiers.

According to the UN:
"A 'child soldier' is defined as any child - boy or girl - under 18 years of age, who is part of any kind of regular or irregular armed force or armed group in any capacity, including, but not limited to: cooks, porters, messengers, and anyone accompanying such groups other than family members. It includes girls and boys recruited for sexual purposes and/or forced marriage. The definition, therefore, does not only refer to a child who is carrying, or has carried weapons" (UN).
Later reports by the UN’s specific children’s fund, or UNICEF titled Children and Armed Conflict 2005 (UNICEF), have estimated that there are approximately three hundred thousand child soldiers at any point in time.  UNICEF also estimated that children serve as combatants in over two-thirds of the (37 out of 55) currently or recently ended conflicts all over the world. 
Although most child soldiers are boys, girls still constitute a significant amount of child soldiers in particular counties or armed groups (Hubertus et al).  So many as 40%, or 120,000 combatants are believed to be female (Breen).  Although many more organizations such as Amnesty International and Human Rights Watch agree upon the round number of 300,000; the amount of child soldiers in the world at one time is impossible to judge.  This is because of the numerous difficulties in what constitutes a child soldier, so much so that there can be no definitive estimate.