From "Trust in testimony and miracles":

I have been of late fascinated by the research of the developmental psychologist Paul L. Harris, who has investigated how young children acquire information through testimony. Harris gauges two psychological hypotheses. The first, which he attributes to Hume, is that children always assess the content of the information: they are more inclined to disbelieve information that widely differs from their earlier experience. The second, which he identifies with Reid's position is that children are naturally credulous; they are inclined to indiscriminately believe what others testify, no matter who they are or what they tell.

...Harris found that children do not fall into either pattern. Pace the Humean account, he found that young children are readily inclined to believe extraordinary claims, such as that there are invisible organisms on your hands that can make you ill and that you need to wash off, and that there is a man who visits you each 24th December to bring presents and candy if you are nice (see e.g., Harris & Koenig, 2006, Child Development, 77, 505 - 524). But children are not blindly credulous either, as Reid supposed. In a series of experiments, Harris could show that even children of 24 months pay attention to the reliability of the testifier. When they see two people, one of which systematically misnames known objects (e.g., saying "that's a bear", while presenting a bottle), toddlers are less likely to trust later utterances by these unreliable speakers (when they name unfamiliar objects), and more likely to trust people who systematically gave objects their correct names (see e.g., Paul L. Harris and Kathleen H. Corriveau Phil. Trans. R. Soc. B 2011 366, 1179-1187.) Experiments by Mills and Keil show that 6-year-olds already take into account a testifier's self-interest: they are more likely to believe someone who says he lost a race than someone who says he won it (Candice M. Mills and Frank C. Keil Psychological Science 2005 16: 385).