The use of child soldiers has troubled human rights activists, policy-makers, and local communities for decades. Although rebellions around the world routinely use children in their activities, many do not. Despite its overwhelming importance for conflict resolution, the topic of child soldiers remains understudied. My research blends classic rational choice and constructivist themes to develop an explanation for when child soldiers will be used, and when they will be avoided. The likelihood of child recruitment is influenced by the value of international opinion; this is determined by the groups' long-term goals. Secessionist rebellions desire to have their own state. However, statehood is jealously guarded by the international community and is only granted under extreme circumstances. The use of child soldiers has been condemned around the world as a crime against humanity, and it can curtail international support. Thus, secessionists should be the least likely rebel type to use child soldiers out of a concern to appear legitimate. Opportunistic rebellions face few constraints in their recruitment efforts. They do not desire international support because their long-term goal is the same as their short term goal: profit. Instead of refraining from using children in order to curry favor with external parties, they will abduct, adopt, and abuse children because they are cheaper to employ than adults. Opportunists are unconcerned with losing legitimacy or reducing the chances of victory. Therefore, they should be the most likely to use child soldiers. Concern for costs can affect all rebels. As duration grows, constraints over long-term legitimacy diminish. Therefore, all rebellions should be more likely to use child soldiers as duration increases. I test my theory quantitatively by looking at 103 rebel groups active between 1998-2008. I explore rebellions in Somalia, Colombia, Afghanistan and Sudan to further elucidate the causal mechanisms. There is considerable empirical support for the theory. These results offer policy-relevant conclusions in the areas of rehabilitation and conflict resolution. More importantly, they offer a workable strategy to curb the use of child soldiers in civil war.