Purpose: The youth mental health crisis highlights an urgent need for competent Child and Adolescent Psychiatrists (CAP). While adult psychiatry has advanced competency-based medical education (CBME) with Entrustable Professional Activities (EPAs) and validated workplace-based assessment (WBA) tools, CAP fellowship training significantly lags, lacking specific EPAs or direct observation tools. We aimed to cultivate fellow growth by developing a novel, feedback-driven WBA program focused on the CAP diagnostic interview to address this critical gap. Methods: We implemented a longitudinal continuity clinic at Zucker Hillside Hospital, Northwell Health, where first-year CAP fellows conducted diagnostic intake evaluations under direct attending supervision. We developed the Child Diagnostic Interview Structured Clinical Observation (Child DI-SCO) tool, assessing core CAP diagnostic skills. Attendings made entrustment decisions and provided structured bidirectional feedback after each observation, fostering repeated opportunities for growth. Six fellows and four supervising faculty completed post-program surveys. Results: Fellows' self-rated skills shifted significantly from novice/beginner to intermediate/advanced across all assessed domains. All fellows found direct observation important for skill improvement and feedback constructive/actionable. All faculty confirmed fellows' longitudinal application of feedback. Faculty overwhelmingly agreed that fellows demonstrated proficiency in CAP diagnostic interview skills. Qualitative themes highlighted valued longitudinal mentorship/clinical development. Conclusions: This program, embedding structured feedback and longitudinal mentorship within a CAP continuity clinic, cultivates meaningful, measurable fellow growth. It addresses a critical CAP fellowship assessment gap, offering a replicable, growth-oriented framework aligned with principles of advancing learner success through feedback, mentorship, and second chances.