An inaccurate perception of motor competence might compromise the engagement of children in physical activities and might be a problem in terms of safety in physical education classes or at playgrounds. The relationship between estimation and actual performance in children with different levels of performance in fundamental movement skills (FMS) was analyzed. Three hundred and three children (aged 6–10 years) were ranked according to their performance in FMS tasks: jumping, kicking, throwing, and walking backward (WB) on a balance beam. Tertiles were created for each task according to children’s performance. Before performing the tasks, children estimated their maximum performance. Absolute percent errors (i.e., deviation percentage from accurate estimations) and error tendency (i.e., frequency of underestimations, right judgments, or overestimations) were calculated. All performance groups tended to overestimate their skills at all tasks, except for the upper tertile group at the WB task (underestimation tendency). After controlling for age, children in the lower tertiles were consistently less accurate than children in the upper tertiles, exhibiting greater absolute percent errors for all the tasks. The overestimation tendency that was found might positively influence children’s engagement in physical activities, but unrealistic estimations might be a problem in terms of safety.