Working memory (WM), a key feature of the cognitive system, allows for maintaining and processing information simultaneously and in a controlled manner. WM processing continuously develops across childhood, with significant increases both in verbal and visuospatial WM. Verbal and visuospatial WM may show different developmental trajectories, as verbal (but not visuospatial) WM relies on internal verbal rehearsal, which is less developed in younger children. We examined complex VWM and VSWM performance in 125 younger (age 4–6 years) and 101 older (age 8–10 years) children. Latent multi-group modeling showed that (1) older children performed better on both verbal and visuospatial WM span tasks than younger children, (2) both age groups performed better on verbal than visuospatial WM, and (3) a model with two factors representing verbal and visuospatial WM fit the data better than a one-factor model. Importantly, the correlation between the two factors was significantly higher in younger than in older children, suggesting an age-related differentiation of verbal and spatial WM processing in middle childhood. Age-related differentiation is an important characteristic of cognitive functioning and thus the findings contribute to our general understanding of WM processing.