Children with Attention Deficit and Hyperactivity Disorder (ADHD) experience a deficit in cognitive processes responsible for purposeful goal-directed behaviors, known as executive functioning (EF). In an effort to improve EF, we are developing TangiPlan-a set of tangible connected objects that represent tasks children perform during their morning routine. We describe the initial stages of a user-centered design process, consisting of interviews with both domain experts and potential users, followed by paper prototyping. Based on our findings, we formulated preliminary design principles for EF assistive technology: facilitate organization, time management and planning; involve caregivers in the process, but strive to reduce conflict; implement intervention techniques suggested by experts; avoid distraction by mobile phones; avoid intrusion. We discuss the benefits of implementing these principles with a tangible interface, present our prototype design, and describe future directions.