Explain that warm air rises in hot clouds and encounters cold air, charging the particles in the cloud making them both positive and negative and this is called static electricity. Explaining static and currents may be skipped for very small children (pre-school) as it may be too complicated and too detailed for them to handle. 'Warm air touches cold air and things begin to happen' is a good introduction for a smaller child.
Tell children that when enough positive and negative charges occur, they build up too much energy and explode in a flash of light that we call lightning. This flash of light helps balance out the number of negative and positive charges in the atmosphere.
Detail the fact that sometimes the flash of light is movement within the cloud and sometimes it is movement between the atmosphere and the ground. Simple sketches as you talk will make things much clearer for your child.
Impart that the movement between the atmosphere and the ground is the reason that tall objects on earth like trees are struck by lightning.
Describe how light travels faster than sound, and because of this, we see the flash of lighting before we hear the clap of thunder. Ensure your child understands the direct relationship of thunder and lightning so they do not see thunder as a separate phenomenon.
Conclude that thunder and lightning lasts only as long as is necessary to get all the electrical charges in the atmosphere back in balance.