Background Sleep problems are common in young people with epilepsy and neurodevelopmental disorders, impacting learning, behaviour and family function. Sleep disturbances can trigger seizures, which in turn disrupt sleep, creating a vicious cycle. Available behavioural interventions have been designed and positively evaluated for typically developing children or selected neurodevelopmental disorder populations, but these do not address epilepsy-specific concerns. Most relevant interventions are delivered face to face by psychologists, limiting accessibility and scalability in the National Health Service. A digital intervention based on behavioural change techniques, adapted for parents of children with epilepsy, is a potential solution. Objectives The overall aim was to broaden the perspective of the paediatric team managing young people with epilepsy beyond a narrow focus on seizures towards issues of importance to young people with epilepsy and their caregivers. To enable this, we (1) developed a core outcome set for childhood epilepsy research that was meaningful to children and their families and recommended candidate epilepsy-specific patient-reported outcome measures of children’s health-related quality of life and (2) produced an online behavioural intervention for sleep problems in young people with epilepsy, which we (3) evaluated using mixed-methods in a randomised controlled trial in the United Kingdom. Design A systematic review and Delphi survey to identify items of the core outcome measure set, with steps toward identifying suitable core outcome measures (work package 1) and a randomised controlled trial with internal pilot (work package 4) of an online behavioural sleep intervention cocreated with parents and young people (work package 3). The trial was evaluated using both quantitative and qualitative methods (work package 2) and with a cost-effectiveness analysis (work package 4). Setting Online and telephone survey of healthcare professionals. Online and in-person meetings of Delphi panel. Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention was created in a university psychology lab. The clinical trial took place in 26 National Health Service paediatric outpatient clinics across the United Kingdom. Participants Eighty-eight healthcare professionals completed a practice survey. One hundred and two stakeholders contributed to the core outcome set. Eighty-five children aged 4–12 years with clinician confirmed epilepsy and parent-reported sleep problems took part in the clinical trial. Interventions Parental access to the Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention online behavioural intervention plus standard care. The intervention arm was compared against standard care alone. Main outcome measures The primary end point was total score on the Child Sleep Habits Questionnaire, 19-item version and incremental cost per quality-adjusted life-year gained. Results Parent and clinician interviews before we designed the program highlighted a mismatch in patient expectations and healthcare providers’ capacity to meet specific health-related needs, including sleep problems. We developed a core outcome set for childhood epilepsy research, with multiple stakeholder input, comprising 10 domains and identified 2 leading candidate epilepsy-specific, patient-reported outcome measures of children’s health-related quality of life (work package 1). We also created an epilepsy-specific, online, parent-based behavioural intervention for sleep problems (Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention) (work package 3). The internal pilot feasibility study demonstrated that parents’ treatment preferences would render a drug arm unfeasible and therefore a factorial drug/behaviour design was simplified to behaviour intervention versus standard care work package 4. Eighty-five children were randomised (42 standard care, 43 standard care +  Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention). The mean Child Sleep Habits Questionnaire score at 3 months did not differ significantly between standard care and standard care +  Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention groups [mean difference = 3.00 (1.46), 95% confidence interval 0.06 to 5.93, p  = 0.05)]. There were no adverse effects in either group. We note that 20/43 families given access to Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention did not engage with the core material. Reasons for this were explored in the qualitative assessments (work package 2) conducted at 3 and 6 months after randomisation. Twenty-two families participated in interviews. While Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention content was described as ‘high quality’, one father reported being overwhelmed by the amount of information and noted that some of it conflicted with other sources. Engagement was further affected by time constraints and distractions. Four Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention users also reported problems with parental sleep disruption and tiredness, including falling asleep, staying asleep, waking up or restless sleep. The economic analysis (work package 4) yielded an incremental cost-effectiveness ratio of £433,167 per quality-adjusted life-year gained and a probability of 0.01 for Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention being cost-effective at a threshold of £20,000 per quality-adjusted life-year. The results of the full trial showed that the Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention behavioural intervention plus standard care was not superior to standard care alone in improving parent report of child sleep problems in young people with epilepsy. However, there was a significant decrease of 16.5 minutes in child sleep onset latency between baseline and follow-up in the Changing Agendas on Sleep, Treatment and Learning in Epilepsy Online Sleep Intervention arm compared to standard care. Parental knowledge of child sleep also significantly increased in the intervention arm. Limitations A significant proportion of participants randomised to the intervention did not engage with the online material. The health economics and secondary outcomes analyses were limited by large amounts of missing data in the final follow-up. Conclusions These results suggest that an exclusive digital approach to solve sleep problems for children with epilepsy, despite objective efficacy, has limited effectiveness and cost-effectiveness as a public health strategy in the National Health Service. Future work Future studies should evaluate a blended intervention of behaviour change techniques coached by paediatric epilepsy specialist nurses. Training in sleep management could be incorporated into secondary care mental health assessment and treatment. Trial registration This trial is registered as ISRCTN13202325. Funding This award was funded by the National Institute for Health and Care Research (NIHR) Programme Grants for Applied Research Programme (NIHR award ref: RP-PG-0615-20007) and is published in full in Programme Grants for Applied Research ; Vol. 14, No. 16. See the NIHR Funding and Awards website for further award information.