The point of a plasma ball, other than the inherent beauty is the ability to safely interact with flowing electricity.  That simple. This is what's inside the wires. This lightning in a bottle. This is the forth state of matter. This is nebulae. It is also an opportunity for enhanced measurement meaning the use of a simple instrument to indicate the existence of something beyond the senses. It's also a toy. It's fun. Have the child touch the ball with her ear or nose. Also point out that LED shoes will blink when she touches the ball.
Then energy transformation is first a change in electricity itself so the transformer to the ball is,
Electricity > Electricity > Motion and Light....
adding the  AC current sensor it continues to
> Electricity > Light and Sound
The current tester blinks and chirps, speed depending on distance. If you hold the tester far enough away to not blink and chirp and then place your oher hand on the ball the tester will blink and chirp meaning that you are in the circuit, part of the field.