Early childhood is a critical time for the development of healthy eating and physical activity (PA) behaviors. Digital behavior change interventions (DBCIs) provide opportunities to overcome barriers presented by traditional parenting interventions to enhance children’s health behaviors. Through the intervention context of the KnowHow Program, a mobile app-based parenting DBCI that aimed to improve preschoolers’ nutrition and PA behaviors, this dissertation elucidated strategies for designing, developing, and implementing DBCIs for parents of preschoolers. Aim 1 presented a user-centered design-based framework (u-CREATE) that serves as a guide for interventionists in designing digital and hybrid interventions. Aim 2 applied the u-CREATE framework in the iterative development and usability testing of the KnowHow app. Through a novel methodology for systematic, early-stage usability testing with usability domains and constructs, Aim 2 identified strategies to develop DBCIs to increase the likelihood that it would be used as intended to maximize intervention fidelity. Aim 3 described the development and testing of the child-centered components of the KnowHow Program, the KidZone and Activity Kits. Development included concept and usability testing with preschoolers and parents, respectively. A pilot test that explored feasibility and implementation outcomes (digital engagement, enactment, barriers and facilitators to use) revealed that while acceptability of the KidZone was high, time was the most prominent barrier in parents’ engagement and enactment of KidZone components. Aim 4 expanded on implementation-related contributions by examining persuasive messaging strategies for promoting engagement in a health behaviors DBCI. This randomized, controlled survey experiment examined the effect of theory-based persuasive messaging strategies on parents’ intentions for PA parenting practices and the psychosocial antecedents (self-efficacy, enjoyment, importance) to these intentions. Results indicated that for male caregivers, messages that described how to engage, as opposed to just why were most impactful and, for all parents, enjoyment mediated the relationship between messaging condition and change in PA intentions. This dissertation showcases the importance of ensuring that DBCI design practices are user-centered, development methods promote DBCI use as intended, and implementation strategies emphasize user engagement in DBCIs, to enhance DBCI’s engagement and fidelity and ultimately, improve their effectiveness in improving health behaviors.