This meta-analysis presents empirical research using a Philosophy for/with Children (P4wC) approach. P4wC involves a teacher engaging in philosophical problem posing and dialogue with students. There has been research focussed on the benefits derived from this practice for students. This meta-analysis calculates the effect size of these benefits that can be calculated using quantitative measures. This meta-analysis contains 62 data sets from 30 studies. P4wC is found to have a significant overall effect size of 0.65. Moreover, subgroup analyses show that when the research focusses on the development of critical thinking, the effect size is substantial at 0.89. This indicates that P4wC has a large impact on the capacity for children and young people to develop critical thinking. Other subgroup analysis also shows that this holds across age ranges. However, it does differ by region, with studies conducted in Eastern countries yielding a higher effect size than Western countries.