The goal of the present study was to examine whether young children benefited from having an explicit midpoint reference on the number line after training on ordinal knowledge. Preschool-aged children (N = 84; M = 53 months old) were exposed to one session of ordinal training or spatial training. The results show that children who received ordinal training showed near transfer to an ordinal task and far transfer to a number line task. The results also show that children showed more improvements in the version of the number line task where an explicit midpoint reference was presented than no midpoint was presented. In contrast, children who received spatial training only showed near transfer to a 2-D mental rotation task. These results indicate that the ordinal knowledge of the number system is crucial for successful estimation on the number line task.