Mental workload are commonly measured by subjective ratings which risks being rationalizations made in hindsight, or using dual-task that interfere with execution of the primary task. We explored the possibility of using pupillometry in the wild to measure the cognitive effort associated with a given task. We conducted an empirical study of 57 children using a printed Booklet and a digital Tablet instruction for LEGO construction while they wore a head-mounted gaze tracker. Booklets caused a particularly strong pupil dilation when encountered as the first media. Subjective responses confirmed the booklet to be more difficult to use. The children who were least productive and asked for assistance more often had a significantly different pupil pattern than the rest. Our findings suggest that it is possible to collect pupil size data in unconstrained work scenarios, providing insight to task effort and difficulties.