Family Size and Longitudinal Outcomes of a Digital-Human Parenting Intervention in Chinese Preschool Families: Secondary Analysis.

Background: Parenting interventions can improve parental and child outcomes across diverse settings. However, less is known about how family size, including the number of children, shapes baseline conditions, and how intervention effects unfold over time. Most studies also focus on average treatment effects, with limited attention to heterogeneity across family contexts and trajectories of change. Objective: This study examined whether the number of children was associated with baseline differences in parental and child outcomes, moderated immediate postintervention effects, and shaped postintervention trajectories over 6- and 12-month follow-up periods. Methods: We conducted secondary analysis of a pragmatic cluster randomized controlled trial evaluating a universal digital-human parenting intervention delivered through the preschool system in China (N=541). Families were categorized by the number of children (1, 2, 3, or more). We examined (1) baseline differences in parental and child outcomes, (2) moderation of intervention effectiveness at immediate postintervention, and (3) trajectories of change over 6- and 12-month follow-up periods using mixed-effects models. Results: Of the 541 enrolled families, 494 were included in the complete-case baseline analysis. Compared with 1-child families, 2-child families, and families with 3 or more children, reported lower levels of baseline early learning and stimulation and proactive parenting, as well as greater endorsement of corporal punishment and higher parenting stress. We found no statistically significant evidence that the number of children moderated immediate postintervention effects. In intervention-group trajectory analyses, 2-child families showed greater improvement in early learning and stimulation at the 6-month follow-up (b=3.966, 95% CI 1.468-6.463). Families with 3 or more children showed a similar pattern (b=5.749, 95% CI 0.536-10.962), although estimates for this subgroup were less precise because of the small sample size. This subgroup also showed larger but more variable reductions in selected child behavioral outcomes over follow-up. Conclusions: Family size might not always be associated with short-term intervention effectiveness but was associated with divergence in longer-term trajectories. These findings suggest that caregiving demands are relevant for the sustainability of intervention effects. By integrating baseline differences, short-term effects, and longitudinal trajectories within a single framework, this study highlights the importance of moving beyond average treatment effects to more dynamic, context-sensitive evaluations. Designing parenting interventions, particularly scalable digital-human programs, that incorporate sustained and context-responsive support may be critical for addressing variation in family structure and enhancing long-term effectiveness.