The COVID-19 pandemic has had many traumatic consequences for the physical and psychological functioning of children and adolescents. Internet-based interventions can reach a large audience and be a potentially powerful resource for promoting well-being among young people. We tested the efficacy of the web application PandHEMOT®, developed ad hoc for increasing knowledge about pandemics, emotions, and emotion regulation. We involved a sample of 147 Italian third and seventh graders. The sample was assigned to an experimental (participating in the training) and a waitlist condition (who participated following a waitlist design). All the participants completed pretest and posttest measures. The intervention was structured into three units. The training took place between November and December 2021. Generalized linear mixed models and linear mixed models revealed that knowledge about pandemics, emotions, emotion regulation, and metacognitive awareness increased for the experimental condition. Moreover, adolescents performed better than children. The findings supported the efficacy of an Internet-based training for increasing children and adolescents’ resilience, according to the standards of evidence-based research.