Statistical probablity is the only thing we can know exactly.Often students get bogged down in the fact that they don't "understand" how and why radioactive elements decay and miss the whole point of this exercise.Furthermore, Parentium and Daughterium are so different in chemical properties that they don't otherwise occur together.If there were such a pair of isotopes, radiometric dating would be very simple.However, the crater number relation must be calibrated against something with a known age.To measure the passage of long periods of time, scientists take advantage of a regularity in certain unstable atoms.
Since God is the Creator of all things (including science), and His Word is true (“), the true age of the earth must agree with His Word.
Imagine we have an undiscovered element, Parentium, that has a radioactive isotope, Parentium-123, which decays to stable Daughterium-123.
This is the only way Parentium-123 decays, and there is no other source of Daughterium-123.
Or you can tell that certain parts of the Moon's surface are older than other parts by counting the number of craters per unit area.
The old surface will have many craters per area because it has been exposed to space for a long time. If you assume that the impact rate has been constant for the past several billion years, then the number of craters will be proportional to how long the surface is exposed.