The current paper reports analyses of the structure of variability in a time-estimation task. Children between 5 and 11 years pressed a button each time they judged that a brief time interval had passed. In two conditions, children either picked their own time interval, their preferred pace, or they were given an imposed pace of 400 ms (2.5 Hz). The resulting trial series were subjected to detrended fluctuation analysis to estimate the complexity of the temporal coordination between child and task. Results show a developmental trend, from an overly random to more clearly fractal performance when the target time interval was predetermined by the experimenter, but not when the target time interval was chosen spontaneously.