When young children recruit others to help a person in need, media reports often treat it as a remarkable event. Yet it is unclear how commonly children perform this type of pro-social behavior and what forms of social understanding, cognitive abilities, and motivational factors promote or discourage it. In this study, 48 three- to four-year-old children could choose between two actors to retrieve an out-of-reach object for a third person; during this event, one actor was physically unable to provide help. Nearly all of children's responses appropriately incorporated the actors' action capacities, indicating that rational prosocial reasoning--the cognitive basis for effective indirect helping--is common at this young age. However, only half of children actually directed an actor to help, suggesting that additional motivational factors constrained their prosocial actions. A behavioral measure of social inhibition and within-task scaffolding that increased children's personal involvement were both strongly associated with children's initiation of indirect helping behavior. These results highlight social inhibition and recognizing one's own potential agency as key motivational challenges that children must overcome to recruit help for others.