Generally, "science" is probably the best answer to the original question. Here are some examples of how scientific truths may be explained to kids:

-The area inside of a circle equals the radius times half the circumference, until somebody proves otherwise. As far as I know, nobody has proven otherwise.

-The earth rotates around the sun, until somebody proves otherwise. As far as I know, nobody has proven otherwise.

-The earth was flat until somebody proved otherwise. The discovery of the earth not being flat involved an application of the scientific method. By the way, that's a good example of why a young scientist should question everything.