We explored the role of parental testimony in children's developing beliefs about the ontological status of typically unobservable phenomena. US parents and their 5- to 7-year-old children (N = 25 dyads) separately rated their confidence in the existence of scientific and religious unobservable entities (e.g., germs, angels), and were invited to engage in an unmoderated dyadic conversation about the entities. Both parents and children were more confident in the existence of the scientific entities compared to the religious entities. Parental religiosity predicted the strength of their belief in the religious entities, and these beliefs were positively associated with their children's judgments in the domain of religion. We coded parental testimony produced during the unmoderated conversation for a number of subtle linguistic cues that convey their confidence and prevailing beliefs in an entity's existence. The results revealed consistent cross-domain differences: parents expressed more "uncertainty," were more likely to mention "variation in people's beliefs" and "make explicit claims about the ontological status" of the religious, as compared to the scientific entities. However, with increasing religiosity, parents produced fewer cues to uncertainty, mentioned belief variation less often, and were more likely to make claims of endorsement when talking about the religious unobservables. Importantly, the pattern of coded linguistic cues in parental testimony was significantly associated with children's ontological judgments. The present findings have implications for understanding the socio-cultural mechanisms by which confidence in the existence of invisible agents and processes develops in childhood.