What cognitive processes influence how well we maintain information in visual short-term memory (VSTM)? We used a developmentally informed design to delve into the interplay of top-down spatial biases with the nature of the internal memory codes, motivated by documented changes for both factors over childhood. Seven-year-olds, 11-year-olds, and adults completed a VSTM task in which they decided whether a probe item had been present in a preceding memory array. Spatial cues guided participants' attention to the likely location of the to-be-probed item during maintenance. We manipulated the memoranda to contain either highly familiar items or unfamiliar abstract shapes. All participants benefited from cues during maintenance, although benefits were smaller for 7-year-olds than for older participants. Critically, attentional benefits interacted with the nature of the memoranda: Better VSTM maintenance was obtained for cued familiar items. Furthermore, attentional benefits for familiar items correlated with validated measures of visual, but not verbal, short-term and working memory span. These data demonstrate that, in addition to the efficiency with which top-down biases operate during maintenance, the available mental codes for to-be-remembered items influence VSTM and differentially so over childhood. Attentional biases during maintenance seem to operate more efficiently on mental representations that are more robust and can be retrieved more easily. More important, this interaction follows a quantitative development. The findings elucidate further the dynamic interplay between attentional control and VSTM across development.