The development of mathematical skills in early childhood relies on number sense, the foundational ability to discriminate between quantities. Number sense in early childhood is predictive of academic and professional success, and deficits in number sense are thought to underlie lifelong impairments in mathematical abilities. Despite its importance, the brain circuit mechanisms that support number sense learning remain poorly understood. Here, we designed a theoretically motivated training program to determine brain circuit mechanisms underlying foundational number sense learning in female and male elementary school-aged children (ages 7-10). Our four-week integrative number sense training program gradually strengthened the understanding of the relations between symbolic (Arabic numerals) and non-symbolic (sets of items) representations of quantity. We found that our number sense training program improved symbolic quantity discrimination ability in children across a wide a range of math abilities including...