Background: Unintentional injuries are a leading cause of preventable death and a major cause of ill health and disability in children under five years of age. A health promotion mobile phone application, ‘Grow up Safely’, was developed to support parents and carers in reducing unintentional injuries in this population of children.

Methods: A prototype of the mobile application was developed to deliver health education on unintentional injury prevention linked to stages of child development. In order to explore the usability of the app and refine its content, three focus groups were conducted with 15 mothers. Data were analysed using thematic analysis.

Results: The majority of participants reported previous use of health apps, mainly related to pregnancy, and recommended by health professionals. The app was considered user‐friendly and easy to navigate. Participants in two focus groups found the app informative, offered new information and they would consider using it. Participants in the ‘young mum's’ group considered the advice to be ‘common sense’, but found the language too complex. All participants commented that further development of push‐out notifications and endorsement by a reputable source would increase their engagement with the app.

Conclusion: The ‘Grow Up Safely’ mobile phone app, aimed at reducing unintentional injuries in children under five, was supported by mothers as a health promotion app. They would consider downloading it, particularly if recommended by a health professional or endorsed by a reputable organisation. Further development is planned with push‐out notifications and wider feasibility testing to engage targeted groups, such as young mothers, fathers and other carers.