Exact arithmetic abilities require symbolic numerals, which constitute a precise representation of quantities, such as the Arabic digits. Numerical thinking, however, also engages an intuitive non-linguistic number sense, the Approximate Number System (ANS). The ANS allows us to discriminate quantities, approximate arithmetic transformations, and estimate quantities, all without counting individual items. Although the ANS does not require language, estimations made by means of the ANS can be expressed with number words or Arabic digits. A connection between the ANS and school math performance has been established. A child's accuracy in mapping from approximate quantities to Arabic digits is associated with children's symbolic math abilities and can also predict their success at learning new arithmetic skills. Here, we explore whether directly training the mapping between estimated quantities and Arabic digits transfers to better math proficiency. The control training was based on discriminating quantity representations, without involving digits. Each of these three-week computer-based trainings were added to the school schedule. We measured improvements in approximate and exact arithmetic after training. Both the experimental and the control group improved in approximate arithmetic performance. However, in exact arithmetic, results show that strengthening the digit-quantity relation improved the 7-year-olds' competence in symbolic additions and subtractions over and above the improvement measured in the control group. Our results speak to the complexity of the factors involved in developing mathematical abilities, making the case that training the mapping from estimated quantities to digits can be particularly effective in improving children's mathematical performance.