Hadad and colleagues (2010) found that the developmental trajectory for processing of illusory contours extends through early childhood, reaching adult-like levels in adolescence. Processing these contours requires interpolation between inducing elements, but assessing interpolation strength has been limited in prior tests (Shipley & Kellman, 1992). In the present study, visual processing of illusory contours in adults and 3- to 9-year-old children was examined using a gaze-contingent visual search task performed using eye-tracking. Participants were presented with inducer arrays; four inducers were arranged to display an illusory square, while others were randomly oriented. Arrays were presented with a set size of 4, 8, or 12 possible target locations. Inducer support ratios (a function of inducer size and spacing) ranged from 0.1 to 0.5. Saccades to the target were used to calculate response accuracy and timing. We also measured search efficiency by evaluating both reaction time across set size to locate the illusory target shape, and the time of participants’ initiation of first saccades directed toward the target-defined area of interest. In a follow-up analysis, we sought to determine whether the illusory shape preattentively captured attention immediately after the initial saccade, particularly in children. Adults were considerably more accurate in detecting the illusory shape overall, and accuracy improved with age in children. Accuracy improved with increasing support ratio and this pattern was similar across adults and children. Adults demonstrated clear levels of preattentive detection in that they produced more first saccades to the target than children at most support ratios. Gaze contingent eye-tracking effectively measures attention processes; therefore, our data provide a more precise description of the development of illusory contour perception. This saccadic analysis extends findings and overcomes previous limitations by highlighting instances of maximum interpolation strength as assessed using eye-tracking and underscores prolonged development of contour perception in children.