Working memory tasks designed for children usually present trials in order of ascending difficulty, with testing discontinued when the child fails a particular level. Unfortunately, this procedure comes with a number of issues, such as limited psychometric qualities for working memory scores, decreased engagement from high-ability children, and large between-subjects variations in number of trials and testing time. To circumvent these problems, the goal of the present study was to demonstrate the feasibility of assessing working memory in children using an adaptive testing procedure. The principle of adaptive testing is to dynamically adjust the level of difficulty as the task progresses to match the participant's ability. We used this method to develop an adaptive complex span task (the ACCES) comprising verbal and visuo-spatial subtests. The task presents a fixed number of trials to all participants, allows for partial credit scoring and can be used with children regardless of ability level. The ACCES demonstrated satisfying psychometric properties in a sample of 268 children aged 8 to 13 years, confirming the feasibility of using adaptive tasks to measure working memory capacity in children. A free-to-use implementation of the ACCES is provided.