Evaluating promisingness of ideas is an important but underdeveloped aspect of knowledge building. The goal of this research was to examine the extent to which Grade 3 students could make promisingness judgments to facilitate knowledge-building discourse. A Promising Ideas Tool was added to Knowledge Forum software to better support knowledge-building discourse. The tool helped students select promising ideas from their group's written online discourse and then aggregate and display selections to support collective decision making regarding most promising directions for subsequent work. Students knew in advance that their selections would influence the direction of group work, and through iterations of procedures came to better understand how individually selected ideas would become the focus of class discussions and next knowledge-building efforts. The basic design was repeated over two cycles of promising-idea selections, discussions, and follow-up activity to refine ideas. Qualitative and quantitative results indicated that students as young as 8 years of age could make promisingness judgments benefiting their community. Through use of the Promising Ideas Tool and discussion based on results from its use, Grade 3 students achieved significantly greater knowledge advances than students not engaged in promisingness judgments and discussions.