This study explored how children's sustained attention develops and the effect of manipulating task parameters on sustained attention. The sample comprised 57 children (5-12 years) who completed CogState and Score! (Test of Everyday Attention for Children). Novel variability and traditional indices indicated rapid development from 5-6 to 8-9 years on all measures and a developmental plateau from 8-9 to 11-12, with growth evident on some measures. Findings suggest that sustained attention improves to age 10, then plateaus with only minor improvements. Further, performance was generally poorer on high load tasks compared to low load, with the same developmental pattern uncovered.