Mobile phone interventions are evidence-based methods for preventing obesity among Latino adults and school-aged children; however, few such interventions exist to improve the obesogenic behaviors of children in the developmentally critical preschool years (ages 2-5). Focusing on this age group is important since over one-quarter of 2- to 5-year-old Latino children are overweight or obese. Moreover, most documented interventions target mothers exclusively, ignoring the influence that other caregivers such as fathers and grandparents have on the environment and the child's behaviors. We describe the development and refinement of a theory-informed mobile phone intervention using an iterative, user-centered approach that supports healthy weight-related behaviors in preschool-aged Latino children by engaging mothers, fathers, and grandparents. The resulting intervention, "Familias Unidas, Niños Sanos" (FUNS), is a culturally centered bilingual (Spanish/English), 12-week, web-based mobile phone intervention grounded in family systems theory. Through three to four weekly text messages and links to web-based interactive multi-media content, caregivers of 2- to 5-year-old Latino children learn evidence-based practices to support specific child behaviors in three domains: healthy eating, media viewing, and physical activity. Development of specific messages is grounded in social cognitive theory. Participants can connect with a virtual coach and other participants. The prototype received high levels of acceptability and usability among members of the target audience and is ready for feasibility testing. The systematic process of development and refinement of the intervention can serve as a model for other mHealth interventions, addressing the ongoing critique of the general lack of theoretical application in such intervention work.