How do children map number words to the numerical magnitudes they represent? Recent work in adults has shown that two distinct mechanisms--structure mapping and associative mapping--connect number words to nonlinguistic numerical representations (Sullivan, J., 2012). This study investigated the development of number word mappings, and the roles of inference and association in children's estimation. Fifty-eight 5- to 7-year-olds participated, and results showed that at both ages, children possess strong item-based associative mappings for numbers up to around six, but rely primarily on structure mapping--an inferential process--for larger quantities. These findings suggest that children rely primarily on an inferential mechanism to construct and deploy mappings between number words and large approximate magnitudes.