Hand hygiene became one of the key measures to prevent COVID-19 infection and the spread of the virus. This, however, is challenging with young children (e.g., primary school). Primary school teachers spend a lot of time supervising and helping children to wash their hands and making sure they wash their hands regularly and correctly. Educating children of the need for handwashing and training them to wash hands for 20 s alone does not motivate young children enough to wash their hands without supervision. We developed WashWall, an interactive smart mirror that uses gamification to motivate young children to wash their hands in an engaging way. We deployed WashWall in a primary school and conducted a user study (N = 14) to evaluate its practicality, effectiveness, and user experience. Results show that children engaged in the activity for a significantly longer duration, enjoyed playing WashWall, and learned the game easily and fast.