Background: Preschool-age children in hospitals are at a high risk of unexpected incidents. Safety incidents in hospitals can cause serious damage to the children. Objective: To determine the effectiveness of the mobile-type mHealth Safe Kids Hospital (SKH) application (app) for the prevention of hospitalized child safety incidents. Design: This study used a three-group, randomized controlled trial prepost design. Setting(s): This study was conducted in the pediatric ward of three general hospitals in Korea. Participants: A total of 124 eligible hospitalized children and their caregivers were enrolled in the study from June to December 2018. Of these, 116 finally participated in the study, and 8 were excluded because they were discharged before the intervention. Methods: Hospitalized preschool-age children and their caregivers were randomly allocated into three groups: experimental group I (n = 39), experimental group II (n = 39), and the control group (n = 38). Experimental group I received the SKH app intervention, the experimental group II received a paper-based intervention, whereas the control group received the usual intervention. Participants' outcomes of awareness, knowledge, and behavior related to hospital safety, were assessed at two time points: baseline and 24 h after the intervention. Results: Hospital safety awareness had a higher increase after intervention in experimental groups I and II than in the control group. Among the four subdomains of hospital safety awareness, there was a significant increase in the scores of experimental group I on three subdomains after the intervention: falls (F = 8.19, p < 0.001), burns (F = 6.73, p = 0.002), and medical devices (F = 6.81, p = 0.002). In hospital safety knowledge and safety behavior, experimental group I had the highest average score after the intervention compared with experimental group II and the control group; however, there was no statistically significant difference in the average score of the three groups. Conclusions: Using the SKH app is easy to attract the interest of preschoolage children and is also easy for nurses to use in clinical trials; thus, it is considered to be a useful educational intervention to prevent safety incidents in clinical fields in future. Clinical Relevance: It is thought to contribute to the prevention of preschool-age children's safety incidents in pediatric wards.