Have kids recall and re-emphasize the basic fact: In chemical reactions atoms are neither created, destroyed, nor changed one into another. They only change partners, so to speak, i.e., which atoms are bonded to which other atoms.
Here we come to something very different. There are situations where the nucleus of an atom does change. This is no longer chemistry; it is another field known as nuclear chemistry or nuclear physics.
Recall/review the concept of isotopes, the idea that the atoms of a given element all have the same number protons—this is what identifies the element—but may have differing numbers of neutrons. Atoms of a given element with differing number of neutrons are called isotopes.
Most naturally occurring isotopes are stable. They occur intermixed with other atoms/isotopes of the element and have no effect on the element’s chemical properties. They are just there.
Here is the crux—there are isotopes that are unstable. The protons and neutrons in the nucleus do not pack together well. They jostle one another and may throw off a a particle or energy to gain better stability. The element with atoms subject to throwing off one or more particles is known as being radioactive. The particles/energy thrown off is known as radioactive radiation.The following video gives some examples of this.