Strategies to promote mental health care help-seeking among children are needed, especially in low-income and middle-income countries and in complex settings. The aim of this trial was to compare a vignette-based, community-level, proactive case detection tool (CCDT) against standard awareness raising for promoting mental health help-seeking among children and adolescents. This stepped wedge cluster randomised trial was conducted in the Bidi Bidi, Kyaka II, Kyangwali, Omugo, and Rhino refugee settlements in Uganda. Community gatekeepers received a 2-day training session on using the CCDT to proactively detect children with mental health concerns and encourage children (or their caregivers) to use the mental health-care service run by Transcultural Psychosocial Organization Uganda. At baseline, organisations implemented routine detection or mental health awareness-raising activities. At cross-over to CCDT implementation, gatekeepers used the tool in their daily activities. The primary outcome was mental health-care service use by children and adolescents. Child population size estimates at the zone level were not available. Therefore, service use was calculated using total population size. We report the effect of CCDT implementation as an incidence rate ratio (IRR), which we produced from a model that accounts for calendar time, exposure time, and person-time. IRRs were estimated for the analysis of effect over time in the per-protocol and intention-to-treat populations. The trial is registered with the ISRCTN registry, number ISRCTN19056780. 28 administrative zones were selected for trial participation by October, 2021. Between Jan 1, and Nov 8, 2022, seven clusters of four zones sequentially crossed over from routine care to CCDT implementation in 1-month intervals. The CCDT was implemented by 177 trained community gatekeepers. In 9 months, 2385 children visited a mental health-care service; of these, 1118 (47%) were girls and 1267 (53%) were boys (mean age 12·18 years [SD 4.03]). 1998 children made a first or re-entry visit to a service; of these, 937 (47%) were girls and 1061 (53%) were boys (mean age 12·08 years [SD 4·06]). Compared to standard awareness-raising activities, CCDT implementation was associated with an increase in mental health-care service use in the first month after implementation (20·91-fold change [95% CI 12·87-33·99]). Despite a slight decline in service use over time in both the CCDT and pre-CCDT zones, CCDT zones maintained a time-average 16·89-fold increase (95% CI 8·15-34·99) in mental health service use. The CCDT enabled community gatekeepers to increase mental health-care service use by children and adolescents. Vignette-based strategies rooted in the community could become a valuable contribution towards reducing the mental health-care gap among children, especially when accompanied by accessible mental health-care services. Sint Antonius Stichting Projects. For the Arabic, French and Spanish translations of the abstract see Supplementary Materials section.