Evaluation of the informativeness of others' testimony is a key aspect of children's social learning. The present study investigates the developmental change in preschoolers' evaluation of the testimony and choice of learning sources based on the informativeness of others' explanations about conjunctive causal events. In two experiments, 3- to 5-year-olds (N = 224) were presented with causal events in which a machine was activated when Block A and Block B were placed on its top. Children then watched two informants providing explanations that vary in informativeness and accuracy (the inaccurate explanations contain non-causal causes). The informative explanations were the same in both experiments (i.e., A and B have to be on the box to make the machine light up). The under-informative explanations omit one of the causes in Experiment 1 (i.e., A has to be on the box to make the machine light up) or contain the logical term "or" in Experiment 2 (i.e., A or B has to be on the box to make the machine light up). Children were asked to rate the quality of the explanations and choose the trustworthy informant and testimony. In both experiments, children as young as 3 rated the informative explanations higher than the under-informative ones. However, only 4- and 5-year-olds rated the under-informative explanations higher than the inaccurate explanations and tended to choose the informative speaker as their learning source, whereas 3-year-olds cannot distinguish under-informative explanations from inaccurate explanations. These findings demonstrate a developmental trajectory in children's appreciation of the informativeness of others' testimony in evaluating the testimony and choosing learning sources.