Kids are bad at lying, at least at first. In the magical fairy dust of the childhood mind, kids initially have a hard time distinguishing reality from fancy, which you can see in their eagerness to engage in imaginative play.
They also perceive their parents to be essentially omniscient, a belief that won't be completely destroyed until the 20-kiloton blast of puberty. The fuse gets lit early, though, around 36 months, when kids begin to realize that parents can't always read their minds. To their delight (or horror), children discover they can give their parents false information without its being detected. Or, at least, they think they can. The child's realization that you can't always read his or her mind coincides with the flowering of something we call Theory of Mind skills.
This timeline suggested to researchers that children have an age-dependent relationship with certain types of moral reasoning, too. There's evidence that kids are born with certain moral instincts, but it takes a while to coax them into their mature form.