School curricula as well as scientific literature emphasize the need of understanding how children navigate with maps. To date, however, there is no empirically validated assessment tool for analyzing navigational map reading competencies that can be used in map-based navigation tasks in the real-world. This paper fills this gap by presenting "GeoGami NMR Assessment," a test for measuring children's navigational map reading (NMR) competencies for educational purpose. Our target group are primary school children aged six to ten. The assessment uses configural and directional map information to assess the learning progress of individual children regarding their "spatial" navigational performance. While we evaluated our GeoGami NMR Assessment for two specific locations, our study design, our script scoring participants' performances as well as the assessment can easily be adapted by researchers and teachers for their own locations. Our item analysis evaluates difficulties of individual tasks. We validate our assessment and demonstrate, that GeoGami NMR Assessment is suitable to distinguish strong and weak navigational map readers.