Co-designing with children in an online environment is increasingly important due to external factors, such as the COVID-19 pandemic, and the diversifcation and inclusion of youth participants. Many prior studies about co-design with youth focus on co-located or asynchronous online sessions. However, conducting synchronous online co-design sessions adds layers of complexity and uncertainty to collaboration. This paper introduces a model explicating factors to consider when co-designing with children synchronously in an online space. We examined ten consecutive intergenerational participatory design sessions online where children (ages 7-11) and adults designed new technologies. Along with highlighting unexpected moments and interactions, we use theories of improvisation to guide our understanding of dynamic situations that are out of the control of researchers. This work contributes to improving theoretical understanding of improvisation as a method of inquiry for co-designing with youth, and ofers practical suggestions for suitable online co-design techniques and implementation.