Civilian or Combatant?: A Challenge for the 21st Century by UN Convention of the Rights of the Child articles 38 Children in Armed Conflicts & 39 Rehabilitation of Victims
Rome Statute of the International Criminal Court “the conscripting or enlisting children under the age of 15 years or using them to participate actively in hostilities in both international and non-international armed conflicts.”
Suggested subject searches to use in SOLO to find books
Child soldiers - Legal status , laws, etc;
Child soldiers (International law);
War victims -- Legal status, laws, etc;
Children (International law);
International Law and Child Soldiers by 
Reimagining Child Soldiers in International Law and Policy by 
Among the Dead Cities : is the targeting of civilians in war ever justified? by 
Treatment of Combatants and insurgents under the Law of Armed Conflict by 
Detention in Non-International Armed Conflict by 
How the Gloves Came Off by 
Commentary on the Third Geneva Convention 2 Volumes Hardback Set by Specific subject searches to use in SOLO
Convention Relative to the Treatment of Prisoners of War (1949 August 12)
Prisoners of war -- Legal status, laws, etc
Many books classified under subject
War (International law)
will probably have useful chapters too.
The legal status of refugees is dealt with in another Law Library guide
For the wider context, the following Bodleian guides may be useful