JMIR Publications

ABSTRACT

Background:

Parenting interventions can improve parental and child outcomes across diverse settings. However, less is known about how family size—particularly the number of children—shapes baseline conditions and how intervention effects unfold over time. Most studies focus on average treatment effects, with limited attention to heterogeneity across family contexts and trajectories of change.

Objective:

Objective. This study aimed to examine whether family size was associated with baseline parental and child outcomes, moderated short-term intervention effectiveness, and shaped longitudinal trajectories of change following a parenting intervention.

Methods:

At baseline, families with more children reported lower levels of early learning and stimulation and proactive parenting practices, alongside higher parenting stress and greater endorsement of corporal punishment, while child behavioral outcomes and caregiver-perpetrated violence were broadly comparable across groups. The number of children did not significantly moderate intervention effectiveness at immediate post-intervention. However, trajectories diverged over time. Two-child families showed the most consistent improvements, whereas families with three or more children demonstrated larger but more variable gains in several child behavioral domains. In contrast, one-child families showed more limited changes across multiple domains.

Results:

At baseline, families with more children reported lower levels of early learning and stimulation and proactive parenting practices, alongside higher parenting stress and greater endorsement of corporal punishment, while child behavioral outcomes and caregiver-perpetrated violence were broadly comparable across groups. The number of children did not significantly moderate intervention effectiveness at immediate post-intervention. However, trajectories diverged over time. Two-child families showed the most consistent improvements, whereas families with three or more children demonstrated larger but more variable gains in several child behavioral domains. In contrast, one-child families showed more limited changes across multiple domains.

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. Clinical Trial: The trial was prospectively registered on the Chinese Clinical Trial Registry (ChiCTR2400081911).