Demanding tasks require a greater amount of effort, in which case individuals are required to alter their energetic-state to a level appropriate to perform the task. According to the Cognitive-Energetic Model (CEM), children with AD/HD are unable to effectively modulate their energetic state, leading to task underperformance. Using an Eriksen flanker task with varying event-rates, the current study compared the ability of typically-developing children and children with AD/HD to modulate their energetic state. In line with the CEM, it was predicted that children with AD/HD would underperform in the fast and slow event-rates. Results indicated that the groups did not differ in commission errors (i.e., incorrect responses). However, children with AD/HD made more omission errors to incongruent stimuli at the fast and slow event-rates, compared to controls. N2 amplitude was significantly larger for the AD/HD than control group in the slow event-rate. It is concluded that the energetic state modulation dysfunction in children with AD/HD results in an inability to attend to the task, as opposed to an inability to perform the task itself. Furthermore, these task performance differences did not manifest in either the N2 or P3 ERP components. Therefore, inattention in children with AD/HD may have its locus in response preparation, as opposed to stimulus processing, but more research is required to validate these conjectures.