This scoping review aims to identify instruments that measure child outcomes assessed in evidence-based family support programs and to investigate reported differences in the magnitude of Cronbach’s alpha by type of study participant (i.e., community, clinical, at-risk samples). We analyzed publications extracted from three databases, which were then narrowed down to 77 articles. The most used interventions were Triple P (23 studies), Incredible Years (13), and Parent Management Training (13) conducted mainly in Europe (35) and (North) America (25). A total of 30 studies were conducted with clinical, 22 with at-risk, and 22 with community samples. The most used instruments with parents as the respondents were the Eyberg Child Behavior Inventory (32), the Strengths and Difficulties Questionnaire (25), and the Child Behavior Checklist (19). The most used instrument with children/adolescents as the respondents was the Child Depression Inventory (5), and for teachers or other professionals it was the Teacher Report Form (9). Regarding Cronbach’s alpha, one-third of the studies did not report any information, one-third yielded mixed findings, and one-third reported good values. Furthermore, it became evident that information regarding Cronbach’s alpha was often incomplete or missing, especially in studies conducted with clinical and at-risk samples. Further research is needed to investigate why there is a bias in reporting Cronbach’s alpha. This work recommends that future studies emphasize the importance of reporting the psychometric properties of the instruments used to be able to properly compare different studies across different populations, especially when used to measure children’s outcomes.