Three to seven percent of pre-schoolers have developmental problems or child psychiatric disorders. Randomized controlled trials (RCTs) indicate that interventions in early childhood education and care improve long-term outcomes of children from disadvantaged backgrounds. It is unknown if effects generalize beyond the well-structured context of RCTs and to children who may not have a disadvantaged background but have developmental problems or psychiatric disorders. We use data from the population-based Norwegian Mother, Father and Child Cohort Study, recruiting pregnant women from 1999 to 2008, with child follow-up from ages 6, 18, and 36 months to ages 5, 7, and 8 years. This sub-study included 2499 children with developmental problems or psychiatric disorders at age five. We investigate the effects of special educational assistance at age five on mother-reported internalizing, externalizing, and communication problems at age eight. We analyse bias due to treatment by indication with directed acyclic graphs, adjust for treatment predictors to reduce bias, and estimate effects in different patient groups and outcome domains with a hierarchical Bayesian model. In the adjusted analysis, pre-schoolers with special educational assistance had on average by 0.1 (0.03-0.16) standardised mean deviation weaker psycho-social difficulties in elementary school. Mean effect sizes varied between groups and outcomes. We estimate positive effects of educational assistance during the transition from preschool to the school years. It should therefore be considered as an intervention for pre-schoolers with developmental or behaviour problems. More research with improved measurements of treatment and outcomes is needed to identify success factors for their implementation.Competing Interest StatementThe authors have declared no competing interest.Funding StatementWe received funding from ADHD Research Network in Norway. https.//oslo-universitetssykehus.no/fag-og-forskning/nasjonale-og-regionale-tjenester/nevsom/adhd-forskningsnettverk Author DeclarationsI confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.YesThe details of the IRB/oversight body that provided approval or exemption for the research described are given below.Regional Committees for Medical Research Ethics - South East Norway https.//rekportalen.no/All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived.YesI understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note. if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.YesData are available upon application to the Norwegian Mother, Father and Child Cohort Study (MoBa) https.//www.fhi.no/en/studies/moba/