The level of mental health literacy (MHL) in adults who work with or care for children is likely to influence the timeliness and adequacy of support that children receive for mental health problems. The aim of this study was to systematically review the literature on mental health literacy for supporting children (MHLSC, recognition/knowledge) among parents and teachers of school aged children (5 to 12 years old). A systematic search was conducted for quantitative studies published between 2000 and June 2021 using three databases (MEDLINE, PsycINFO and ERIC) and relevant citations reviewed in Scopus. To be included, studies needed to measure at least either ‘mental health knowledge’ or ‘recognition’. Synthesis proceeded according to study design, adult population, child MHP, then MHL outcome. Study quality was assessed using AXIS. 3322 documents were screened, 39 studies met inclusion criteria. 49% of studies examined teachers’ knowledge or recognition of ADHD; only five studies reported on parent samples. Synthesis found a nascent field that was disparate in definitions, methods and measures. Little research focussed on knowledge and recognition for internalizing problems, or on parents. Methods used for measuring knowledge/recognition (vignette vs screening) were associated with different outcomes and the quality of studies was most often low to moderate. Adults appear to have good recognition of childhood ADHD but their knowledge of internalizing disorders is less clear. Further research is required to develop standard definitions and validated measures so gaps in MHLSC can be better identified across populations who have a role in supporting children with their mental health.