Imagine a microscope with variable and nearly unlimited magnification (a few million can be considered unlimited for this purpose.)
Then take a normal tooth, point this microscope at it and zoom in, zoom in more, and then some…
It is fascinating how this hard piece of material takes on intricate structures and showing the fissures and cracks in there to your child might convince them that there are indeed areas were ‘bugs’ can hide and that it might be good to brush those out of these cracks.