The aim of this study was to model the most crucial numerical factors to the development of mathematical skills among children aged five to eight years (i.e. kindergarten, preschool, first and second graders). We categorised numerical skills into four main groups based on the results of longitudinal studies. A series of analyses of test batteries designed to measure the development of mathematical skills in children yielded results in support of this construct. Based on our findings we propose a working model for teachers of core numerical skills that focuses on four major factors: (1) symbolic and non-symbolic number sense; (2) understanding mathematical relations (early mathematical-logical principles, arithmetic principles, mathematical operational symbols, place-value and base-ten system); (3) counting skills (knowledge of number-symbols, number word-sequence, enumeration with concrete objects); and (4) basic skills in arithmetic (arithmetic combinations, addition and subtraction skills with number symbols). The adoption of this working model could improve the efficacy of the assessment of numerical skills of children, but also help educators to structure their support more comprehensively.