The point for the child is that white light can be spread into a Spectrum of colors and that there are colors on both ends of the Spectrum, these being infrared and ultraviolet, which we can't sense, although many plants and animals can. We can, however, use and experience these parts of the spectrum every day with commonly available tools.
We all know what this chart shows.
The question is, how do we show it?
This concrete, physical experience will be the basis of the child's understanding that the electromagnetic spectrum goes beyond what we can see in both directions.
The concept also applies to Sound.
Elephants hear infra sound with their feet.
We see our not yet born children with ultrasound.
The easiest, most accessible source of a spectrum under natural light conditions in a typical classroom setting is a CD.
The second is a piece of diffraction grating.
A round vase can be used to explain how millions of raindrops generate a rainbow.
Soap bubbles work too.
Triangular prisms come in last and a bit cranky as they are brilliant in direct sunlight but difficult to use with an artificial light source suitable for a primary classroom. The problems inherent in full spectrum white light sources being the power requirements, the electrical cords, heat and the fact that the idea is to put the light source in the child's hand not in some fixture she isn't allowed to touch. Hence a variety of flashlights, with the understanding that different flashlights will produce different parts of the spectrum depending on the frequencies they emit. Different flashlights give different results. Flashlights can be combined with lenses and prisms to alter the effect.
One particularly useful tool is a dichroic prism, which separates white light into red, green and blue, the opposite of mixing.