This contribution gives exemplary insight in some results of the project EfEKt. The study evaluates different aspects of the implementation of the tablet app MaiKe, which has been developed to support mathematical competencies of children aged 4–6. The use of digital media in early childhood education is often discussed very critically in Germany. The research design, therefore, compares differences between free and supervised settings of use implementation in two designs for intervention groups. Above all, EfEKt investigates the effect of the app use on the development of children’s mathematical competencies. Our results show that a free play setting does not lead to an extensive use of digital media. The pre- and post-test results show statistically relevant positive effects of the MaiKe app use on the development of mathematical competencies in both intervention groups compared to a control group. Qualitative analyses indicate that the participating children use more sophisticated mathematical strategies over the course of the intervention period.