Despite an increasing use of screens among preschool children and evidence suggesting potential adverse effects, there is a paucity of longitudinal research that aims to disentangle the multifaceted components of screen use and their unique associations with development. We present a protocol for a large-scale national longitudinal study with repeated measurements in Danish preschool children, with the aim of investigating the cross-sectional and cross-lagged longitudinal associations between screen use and psychological outcomes.

The Digital Child Study is a national prospective observational cohort of Danish preschool children. Baseline parent-report data collection commenced in 2024 via online questionnaires, and in total will include three time points over 1 year: baseline (age 4 years), and follow-ups at 6 and 12 months (ages 4.5 and 5 years). Participants were divided into two waves based on birth dates, starting in March and September 2024. Recruitment targeted parents and primary caregivers of all Danish children born between specific dates in 2020. Of 30 235 children whose parents were sent invitations, baseline questionnaire data were available for 11 690 (39%).

Children’s screen use was measured by detailed information of amount, content and timing of children’s screen use, and the broader context, incorporating parental mediation strategies, attitudes, motivations and practices. Cognitive and socioemotional developmental outcomes were measured using validated tools such as the Strengths and Difficulties Questionnaire, the Nordic Five-to-Fifteen parent questionnaire and the Behaviour Rating Inventory of Executive Function—Preschool Version. Questionnaire data will be linked to national social and health registries to enable long-term follow-up. Statistical analyses will include longitudinal modelling to explore associations between screen use and developmental outcomes, with sensitivity analyses for robustness. The study’s large sample size provides high statistical power to detect meaningful effects.

The study adheres to ethical research guidelines, ensuring voluntary participation, confidentiality and compliance with data protection laws, with approvals from relevant authorities. Findings will be disseminated through peer-reviewed publications, conferences and plain-language summaries to engage stakeholders and the broader community.