Our study investigated the knowledge 13 elementary teachers gained implementing a student-centered curriculum in the context of district-wide reform. Participants comprised all the teachers in grades three, four and five at a single elementary school. We believed that investigating teachers' responses to fictional pedagogical scenarios involving nonstandard algorithms would yield insights into critical components of their knowledge base. We looked in particular at teachers' knowledge of "children's mathematics". We found that teachers were in the midst of creating a knowledge base focused on children's mathematics and grounded in knowledge about alternative conceptual trajectories through the elementary curriculum. Teachers' knowledge of nonstandard strategies supported by the curriculum materials was stronger and more coherent than their knowledge of students' novel nonstandard strategies. Strong mathematical knowledge was not necessarily associated with strong knowledge of children's mathematics. Teachers' thinking varied by a topic's treatment in the written curriculum materials used, suggesting implementation of the curriculum as a source of learning.