Background: Among the different modalities of family support, home visiting programs have proved to be one of the most effective secondary prevention models for families involved in child welfare services. Despite the promising outcomes and the extensive available literature, further research is needed to investigate target population characteristics and implementation factors that may explain the heterogeneity in the outcomes of these programs for families receiving child welfare services. Objective: This longitudinal study explored the role of dosage and family characteristics related to the effectiveness of the Family Intervention Program (FIP), which is a home-based Spanish child welfare service for supporting families at psychosocial risk. Method: To this end, a quasi-experimental design was followed, including a time-series evaluation of the 289 families receiving the FIP. In order to examine the impact of the FIP, the evolution of child well-being (CWB) was evaluated with Child Well-Being Scales every 6 months. Results: The results showed that the highest average CWB score was observed after 39 months of intervention, observing no improvements in CWB from that point. Similarly, the results showed that the FIP was more effective for single parent families at higher socioeconomic levels. Conclusions: This article provides new knowledge for the understanding of home visiting programs success, concluding that the programs are not equally effective for all family profiles, and that it is necessary to adjust each program to the specific characteristics of the target population.