Conditional cash transfer programs (CCTs) have shown mixed effects on child development outcomes in the context of poverty. Direct parenting support integrated with CCTs may improve the effectiveness of CCTs for children’s development, and benefits could occur via improvements in parenting practices or the home environment. Here, we use data from a randomized effectiveness trial to examine the pathways connecting parenting support and child development. The parenting program (EI: Educación Inicial) was implemented at scale among beneficiaries of Prospera (a CCT, previously Oportunidades and Progresa). Participants included children between the ages of 3 and 5 years old (n = 1,362) from 91 rural communities in three Mexican states. Communities were stratified by indigenous classification and randomized to one of three arms: (T₀) Comparison group (CCT benefits only); (T₁) CCT benefits plus availability of EI in the community; or (T₂) CCT benefits plus promotion and encouragement by the CCT program to participate in EI. Findings were that participation in the T₂ arm of the study was associated with a 13% increase in the number of play activities that parents engaged in with their children, particularly shared storybook reading and singing. Parents in T₂ showed nearly two times greater odds of reading daily to their children. In mediation analyses, the amount of play activities and shared book reading explained up to 32% of the effects of the EI parenting program on child development outcomes. In this study, collaboration and integration of two social programs was critical for program impact, which occurred through pathways relating to parental engagement in reading and play.