Bilingual children's dominance can predict a variety of language and cognitive outcomes. The purpose of this study was to test the validity of parents' classifications of 3- to 6-year-old bilingual children's dominance against relative receptive vocabulary scores. In Study 1, the parents' classifications of Mandarin-English bilingual children's dominance corresponded to relative vocabulary scores only for children reported to be very Mandarin dominant. Similar results were obtained in Study 2 with French-English bilinguals and very French dominant children. We argue these results reflect well-known estimation processes and that asking parents to judge each language separately might yield more valid results.