Parents' low health literacy (HL) has negative impacts on child health. Parental interventions may improve parents' HL and thus impact child health positively. This study aimed to gain knowledge about associations between parents' comprehensive HL (CHL) and child health after an extended postnatal home visiting program in Swedish multicultural, disadvantaged settings compared to parents receiving regular child healthcare (CHC). This quasi-experimental study used a case-control sampling method to recruit first-time parents through two CHC centres in Stockholm (2017-2020). Participants (N = 151) were interviewed twice through structured questionnaires when their child was <2 months and 15-18 months old. HLS-EU-Q16 assessed parents' CHL. Children's medical records (0-18 months) were reviewed regarding breastfeeding, children's exposure to smoking, language development and healthcare utilisation. Data were analysed with regression models and non-parametric tests. No significant association was found between parents' CHL and child health. However, significantly fewer unplanned visits to the CHC centre were observed among children (0-18 months) in the intervention group irrespective of CHL, compared with children to parents with improved CHL in the comparison group (F = 3.856, p = 0.011). Postnatal home visiting interventions practicing proportional universalism and family-centred care may reduce unplanned visits within CHC in disadvantaged settings despite parents' CHL. Further studies with long-term follow-up are suggested to explore associations between parents' CHL and child health. As a clinical study (not a clinical trial) with appropriate ethical permission with participants' consents, this study was retrospectively registered (18 February 2020) in the ISRCTN registry (ISRCTN10336603).