Randomised controlled trials are often inappropriate for many forms of preventative children’s services; as such, observational studies using administrative data can be valuable for evidence-based policymaking. However, estimates of effectiveness can be confounded by differences in thresholds of intervention and national policies that exert pressure on local trends. This study adjusted for these factors using methods developed in clinical psychology to control for individual traits and developmental trajectories, Autoregressive Latent Trajectory Models with Structured Residuals, to analyse the relationship between local authority preventative spending and Children in Need (CIN) rates in England. Higher spending was associated with significant decreases in CIN rates between 2010/11 and 2014/15, but not from 2014/15 onwards. In the first half of the decade, 1% increases in expenditure were associated with between 0.07% and 0.157% decreases in CIN rates. Based on average local authority spending cuts, this translates to an additional 13,000 to 16,500 children and young people put or kept at risk of developmental or health impairments nationally for each year between 2011 and 2015. These findings highlight the potential of early help/family support policies and concerns around how their effectiveness has changed consequent to prolonged austerity and a deliberate policy focus on ‘what works’.