The purpose of this scoping review was to explore operationalizations and related conceptualizations of young children’s engagement in early childhood education and care (ECEC) settings. The literature search was conducted in March 2021 across ERIC, PsycINFO, Scopus, and Web of Science databases, with the aim of identifying studies where child engagement or involvement in ECEC settings was quantitatively assessed. The search resulted in 5965 articles, of which 286 were included in this review. Data were extracted about engagement conceptualization, theoretical frameworks, study population, study design, and engagement measurement tools and methods. Findings show variations in definitions and measurement of child engagement. Almost two-thirds of the studies lacked an explicit definition of child engagement. Young children’s engagement was typically defined as behaviors and interactions with the social and material environment, while involvement was depicted as an internal experience. The most common method of measuring children’s engagement in ECEC was observations by an external observer, followed by teacher surveys. Seventy-seven unique established measures of child engagement were identified. About one-third of the identified studies relied on unestablished measures of child engagement. Measures of general child engagement in ECEC had a focus on behavioral aspects of engagement, whereas most measures with a focus on engagement in academic activities also included cognitive and emotional aspects. To advance the research of child engagement in ECEC settings, more attention should be put into clarifying the concept of child engagement in terms of its generalizability, specificity, and temporality. Corresponding operationalizations should be precisely described. Our recommendations also include validating existing measures of child engagement and developing self-reports for young children.