The purpose of the inquiry was to understand how children in primary grades (i.e., preschool, kindergarten, and second grade) engaged with affordances and constraints of features in a Base-10 Blocks virtual manipulative mathematics app designed to promote learning opportunities. Researchers conducted one-to-one interviews with 100 primary grade children as they interacted with the Montessori Number Base-10 Blocks iPad app. The video data were qualitatively analyzed using open descriptive, thematic, and structured coding. Results show that children's actions when interacting with app features can affect their engagement with designed mathematics constraints and affordances. Results also identified three emergent themes around children's engagement with the app's simultaneous linking features: verification, self-correction, and making connections. Findings suggest the importance of helping children identify and reflect on affordances within virtual manipulative mathematics apps. These findings also indicate that as designers, educators and researchers design or select virtual manipulative mathematics apps for classroom use; they should to consider children's prior achievement as well as in-app perceptions and engagement with design features in the apps.