When viewed under a microscope (see text for explicit instructions) fine particles are seen to jiggle about. By all means, help kids observe this for themselves and have them ponder about what causes the jiggling. The explanation is that atoms, hence molecules, undergo continuous motion, i.e., they have kinetic energy due to temperature (heat) of the surroundings. Hence, all surfaces are continually bombarded by surrounding water molecules. Fine particles are knocked about one way and another by this bombardment. Larger particles and objects are not seen to move because their larger size results in all sides being bombarded (pushed) equally.
In turn, Brownian motion provides an explanation of why diffusion occurs. Please post questions and let us know your kids responses.
The following site provides a video of Brownian Motion