Collective efficacy refers to a perceived sense of connectedness and willingness to intervene among youth, and is a potential aspect of positive youth development (Larson in Am Psychol 55:170–183, 2000; Lerner et al. in Child Dev 71:11–20, 2000; Sampson et al. in Science 277:918–924, 1997). Theoretically, those who feel connected to a group that is empowered to positively influence the behavior of their peers may demonstrate fewer problem behaviors. Few studies, however, have measured the impact of youth perceptions of collective efficacy. As a relatively new child-related research topic, there is much to be learned. One contribution to the foundation of this research agenda begins by evaluating the reliability and validity of a measure of collective efficacy with elementary children attending community-based afterschool programs. This paper describes the internal consistency reliability and various indicators of construct and concurrent validity of the Collective Efficacy Among Children Scale. The measure was found to have high internal consistency reliability. Construct validity was tested using exploratory factor analyses of collective efficacy including the dimensions of willingness to intervene and cohesion found in previous research (Sampson et al. in Science 277:918–924, 1997). Concurrent validity assessed relations between the scale and other measures in theoretically congruent ways. Using Hierarchical Linear Models to account for children's nestedness in after-school programs, connectedness was found to be more related to emotional adjustment, particularly children’s prosocial attitudes (caring about others and sharing). Children’s perception of the willingness of the group to intervene was found to be related to less problem behavior, (i.e. smoking tobacco, drinking alcohol, vandalism, and stealing). The implications suggest that future research should further explore children’s collective efficacy, and ways to foster its development in youth-serving afterschool settings.