Indoor air quality (IAQ) is especially important for children because they are more susceptible to the deleterious impacts of poor air quality compared to adults. While devices to monitor IAQ are increasingly available, these are designed primarily for adults, and little attention has been paid to their potential use by children. This paper describes an effort to engage children directly in the design of an IAQ visualization interface for children. In engaging children in participatory workshops, we found that they rely heavily on visual, olfactory, and thermal cues to perceive and assess IAQ. Reflecting on these findings and based on design principles for technology for children, we created and tested child-friendly interface prototypes for IAQ visualization. Based on children's input, we designed a final set of visual interfaces that will be implemented in the IAQ monitor. The next study will test and deploy the monitor in the real world.