It's difficult to tell whether a kid is lying. Many kids lie recreationally, just because they're in the mood to make something up or report an interesting happening that didn't happen. But it would be remiss to assume that kids lie more often then they tell the truth. If you assume the worst when a child is telling the truth, it instills in them that nobody will believe them when something bad happens, and that mindset can have catastrophic consequences.
I think the best practice when doubting a kid's credibility is to consider the likelihood of their claims. Is it more likely for an elementary school to hire a substitute teacher who doesn't speak any English, or is it more likely for your 8-year-old to lie about it for the plot? I'd say the latter. Is it more likely for a 7-year-old to steal a puppy from their neighbor's house or for a puppy to wander into their front yard? That question is one the dad in this story answered incorrectly.
This image is for illustration only, and the subjects are models; the image does not depict the actual subjects of the story.