Introduction Small efficacy trials have demonstrated that multicomponent interventions can improve early child development. We evaluated the large-scale delivery of a multicomponent intervention delivered by government health workers throughout a rural subdistrict in northwestern Bangladesh. Methods We evaluated a group-based, multicomponent intervention with a curriculum covering responsive parenting, caregivers' mental health, lead exposure prevention strategies at the household level, water, sanitation, hygiene and nutrition. Group sessions were held throughout a rural subdistrict of Bangladesh (August 2019-March 2020). A longitudinal sample of caregivers (n=517) of children 6-24 months was assessed at baseline and endline (primary cohort), and 1179 additional caregivers were assessed only at endline (supplementary cross-sectional). Outcomes were the variety of child play activities and materials, number of books, caregiver depressive symptoms and nutrition and lead knowledge. For primary analyses, we used difference-in-difference. Results Over half (n=276, 53%) of the cohort participants attended any of the 16 intervention sessions and of these, 83% (228) attended 2+. Caregivers attending 2+ sessions, compared with ≤1 session, had more play materials (adjusted mean difference: 0.58; 95% CI: 0.30, 0.85) and were more likely to have any children's books (adjusted prevalence difference (aPD): 0.26; 95% CI: 0.18, 0.34), to have heard of lead (aPD: 0.13; 95% CI: 0.07, 0.19) or to know how to avoid harm from lead (unadjusted PD: 0.13; 95% CI: 0.08, 0.17). These findings were similar to those from the supplementary cross-sectional analysis. There were no differences in caregiver depressive symptoms in either analysis. More child play activities and nutrition knowledge were associated with attendance in the cross-sectional sample. Conclusions A multicomponent child development intervention delivered by government health workers increased the presence of children's toys and books and caregiver knowledge of lead in families who attended two or more sessions. Further adaptation and alternative delivery methods are likely to improve the reach and the breadth of impacts.