Previous studies have linked preschoolers’ 24-hour movement behaviours with fundamental motor skills (FMS), but often examined behaviours separately, neglecting their compositional nature. This study investigated associations between daily movement behaviour composition and FMS, estimated theoretical changes in FMS with time reallocations, and tested whether associations differed by sex. In this cross-sectional study, 270 children (3–6 years) wore accelerometers to assess light and moderate-to-vigorous physical activity (LPA, MVPA) and sedentary behaviour (SB). Sleep duration was parent-reported. FMS was measured using TGMD-3. Compositional linear regression analysis examined associations between daily movement behaviours and FMS, and compositional isotemporal substitution estimated theoretical changes in FMS under time reallocations, accounting for the compositional nature of behaviours. 24-hour movement behaviours composition was significantly associated with FMS. Reallocating 30 minutes from SB to MVPA was associated with higher total motor skills scores (2.997 raw scores, ~3% of the TGMD-3 total scale). The same reallocation was also associated with higher locomotor skills scores, but no changes in object control skills. These associations were asymmetrical, reflecting a non-linear association; for instance, an additional 30 minutes of MVPA was linked to smaller increases (+2.997 raw scores) than the decrease observed for equivalent reductions (–4.007 raw scores). Model-based projections show that reallocating SB to MVPA was associated with higher total motor and locomotor skills scores, with patterns more apparent in boys, while links with object control skills were less consistent, underscoring domain- and sex-specific associations.