Background: A high level of food cue reactivity (FCR) is a risk factor for overeating and weight gain. This randomized trial investigated whether open-label placebos (OLPs: placebos honestly administered) can reduce FCR (reported appetite) in children and adolescents. Method: Children (n = 73; 8–12 years old) and adolescents (n = 187; 16–18 years old) were randomly assigned to either an OLP group or a control group (without OLP). Participants viewed images depicting food (sweets and fruits) and non-food items. Before viewing, the OLP group received a placebo for appetite reduction. Participants rated their desire to eat the depicted food items (0–100) and the perceived effectiveness of the OLP intervention. Results: The OLP exhibited a large effect in children, leading to a general reduction in appetite (group difference OLP vs. no OLP: M = −20.8; ηp2 = 0.20). This general effect was absent in adolescents, whose appetite reduction was restricted to fruits (OLP vs. no OLP: M = −8.1; ηp2 = 0.03). Additionally, children perceived the OLP as more effective than adolescents. Conclusion: The reduced response and skeptical attitudes of adolescents towards OLP treatment require further investigation.