BACKGROUND: We investigated the processing of task-irrelevant and unexpected novel sounds and its modulation by working-memory load in children aged 9-10 and in adults. Environmental sounds (novels) were embedded amongst frequently presented standard sounds in an auditory-visual distraction paradigm. Each sound was followed by a visual target. In two conditions, participants evaluated the position of a visual stimulus (0-back, low load) or compared the position of the current stimulus with the one two trials before (2-back, high load). Processing of novel sounds were measured with reaction times, hit rates and the auditory event-related brain potentials (ERPs) Mismatch Negativity (MMN), P3a, Reorienting Negativity (RON) and visual P3b. RESULTS: In both memory load conditions novels impaired task performance in adults whereas they improved performance in children. Auditory ERPs reflect age-related differences in the time-window of the MMN as children showed a positive ERP deflection to novels whereas adults lack an MMN. The attention switch towards the task irrelevant novel (reflected by P3a) was comparable between the age groups. Adults showed more efficient reallocation of attention (reflected by RON) under load condition than children. Finally, the P3b elicited by the visual target stimuli was reduced in both age groups when the preceding sound was a novel. CONCLUSION: Our results give new insights in the development of novelty processing as they (1) reveal that task-irrelevant novel sounds can result in contrary effects on the performance in a visual primary task in children and adults, (2) show a positive ERP deflection to novels rather than an MMN in children, and (3) reveal effects of auditory novels on visual target processing.