Cognitive control can be triggered by explicit or implicit events; it has been proposed that these two possibilities tap into dissociable mechanisms. In this study, we investigate this idea by testing whether young children, who struggle with explicitly-triggered control, can demonstrate proportion congruency effects - which are based on implicit learning of task regularities and thus indicative of implicitly-triggered control. In a series of five experiments, preschoolers indeed demonstrated significant proportion congruency effects, including both list-wide proportion congruency (LWPC) and item-specific proportion congruency (ISPC) effects, in a Stroop-like task and in a flanker task. These effects did not increase with age, contrary to what is typically observed for explicit control. These results demonstrate that young children show early evidence of cognitive control - including proactive control - when it is triggered by implicit events, at an age where explicit control, and particularly proactive control, is not yet functional. By showing evidence of an early ability for fine-grained adjustments of cognitive control when control cues are learned implicitly, these results support the proposed functional dissociation between explicit and implicit cognitive control.