Background: Early childhood obesity is an urgent public health challenge, with long-term health risks. The 12-month fruehstArt intervention aims to improve healthcare for 3 to 6-year-olds with overweight and obesity in Germany through a family-centred approach, including home-based counselling with coaches, paediatric consultations, and a supportive web application for both German- and Turkish-speaking families. This process evaluation will examine the barriers and enabling factors critical for successful implementation, identify necessary adaptations to the intervention, and assess its quality and acceptability among families, coaches, and paediatricians. Methods: This formative evaluation will use a mixed-methods approach including qualitative and quantitative data. Semi-structured interviews will be conducted with parents, coaches, and paediatricians at two different time points. Interviews will be analysed using qualitative content analysis. An Implementation Quality Index assessing the four dimensions of dosage, adherence, quality of delivery, and participant responsiveness will be developed, based on data collected from coaches carrying out the home-based counselling and participating parents of the intervention group. Linear mixed models for repeated measures will be used to analyse the relationship between data of the Implementation Quality Index and the primary and secondary outcomes of the effectiveness evaluation. Conclusions: The formative evaluation of the fruehstArt intervention is expected to identify important determinants of the implementation and provide valuable insights for future strategies to improve implementation. By identifying barriers and facilitators to participation, this study aims to pave the way for an effective dissemination of the intervention and ultimately contribute to improved health outcomes for children.