Childhood anxiety is a problem not only because of its negative consequences on the well-being of children but also because of its adverse effects on society and its role in mental disorders later in life. Adequate prevention might be the key in tackling this problem. The effectiveness of Coping Cat, as an indicated CBT-based prevention program in Dutch primary school children, was assessed by means of a randomized controlled trial. In total, 141 children aged 7–13 with elevated levels of anxiety and their mothers were included and randomly assigned to an intervention group and a waiting list control group. After screening, all participants completed baseline, post-intervention, and 3-month follow-up assessments. The results showed that Coping Cat, as an indicated prevention program, reduces children’s self-reported anxiety symptoms, with Cohen’s effect size d of 0.66 at the 3-month follow-up. A moderating effect was found for baseline anxiety level; specifically, children with high levels of baseline anxiety who received the Coping Cat program had lower anxiety levels at follow-up compared to children with high levels of anxiety in the control condition. No moderating effects of gender or age were found. An unexpected decline in anxiety levels from screening to pre-assessment was found in both groups, and this decline was stronger in the experimental group. These promising results warrant the implementation of Coping Cat as an indicated prevention program.