Mathematically able and less-able children performed a series of complex working memory tasks, in which attention was regularly switched between the tasks’ processing and storage components. Complex span was measured under six conditions designed to vary processing pace and processing complexity. Processing complexity corresponded to the retrieval demands of three different processing activities (articulatory suppression, reading numbers and simple additions), each of which was presented at a slow and fast pace. Overall, the children’s span performance declined as processing pace and complexity increased. The spans of the mathematically less-able children were lower than those of the able mathematicians’ across all levels of pace and complexity. There were no interactions between mathematical ability group and processing complexity, or pace, which indicated that, contrary to expectations, the less-able and able mathematicians were influenced by increased cognitive load in similar ways. The overall findings were consistent with the predictions of the time-based resource-sharing model of working memory, which states that complex span performance is disrupted by processing activities that divert attention away from the maintenance of items in storage. The observed working memory deficit of the less able mathematicians children is discussed in relation to similar findings in the field of mathematical learning difficulties and in relation to research in the related domains of working memory, executive control and attention.