Substantial research has aimed to characterise and measure early childhood education and care (ECEC) quality. However, heterogeneity in measures, methods and contexts across studies has made it difficult to reconcile the inconsistent associations reported between quality dimensions and child outcomes. While there is broad consensus that early interactions and experiences may be most strongly and directly influential to children’s developmental progress, attempts to identify aspects of quality interactions that relate most highly to child outcomes have tended to focus on particular measures and/or contexts. The aim of this systematical review was to reconcile the available evidence on associations of dimensions of quality interactions in formal ECEC settings (adult–child interactions and associated context and content) with the outcomes of children aged 3–5 years. Uniquely, this review examined how rates of significance differed by measure, country and study characteristics (e.g. sample, study design, risk of bias) – providing nuanced insights on quality indicators that most reliably account for children’s developmental progress. Seven databases were searched for the years 2000–2022, yielding 90 studies reporting 870 associations of interaction quality with various child development and educational outcomes. Results indicated little evidence for global ECEC quality indices (e.g. those integrating process quality indicators with structural elements) relating to child outcomes. The consistency in patterns of association improved for some dimensions of interaction quality (e.g. supporting play), with other dimensions showing low support even when they aligned with the outcome (e.g. instructional support with cognitive-academic outcomes). By providing an overview and reconciliation of evidence on the child-level associations in ECEC quality, across diverse measures and contexts, this review raises important questions of current ECEC quality assumptions and practices.