Children's estimation skills on a bounded and unbounded number line task were assessed in the light of their familiarity with numbers. Kindergartners, first graders, and second graders (N = 120) estimated the position of numbers on a 1--100 number line, marked with either two reference points (i.e., 1 and 10: unbounded condition) or three reference points (i.e., 1, 10, 100: bounded condition). Estimations were more accurate and less variable in older compared to younger children and, beyond age, in children who were familiar with larger numbers. The number of reference points yielded neither a main effect nor an interaction effect with familiarity. Our results underline that children's familiarity with numbers is a pivotal factor for the quality of number line estimations that might even obliterate potential effects of additional reference points.