Young children can engage in diagnostic reasoning. However, almost all research demonstrating such capacities has investigated children's inferences when the individual efficacy of each candidate cause is known. Here we show that there is development between ages five and seven in children's ability to reason about the number of candidate causes whose efficacy is unknown (Study 1). We also find development between ages six and seven in these abilities when children are presented with several uncertain candidate causes in an additive causal system (Study 2). These findings demonstrate how children's diagnostic reasoning abilities develop beyond the preschool years and illustrate possible relations between children's developing diagnostic inference and scientific reasoning capacities.