Well, you can start by telling her that when you start a fire in your fireplace, the wood burns in the air and gives off light (And heat). What if you could get wood to burn without giving off a lot of heat? That's what scientists did when they invented "glow-sticks". It's not wood inside that's burning, but it's two chemicals that, when mixed, glow and give off light, just like the wood in your fireplace glows and gives off light and heat. Of course, the scientists had to find the right chemicals to mix, chemicals that wouldn't give off much heat (that our body could detect anyway). So, when you snap a "glow-stick", it's really not "chemically" much different than lighting a fire. It is however much safer.
If you need more info, I've taken the text below from a website
http://www.sciam.com/askexpert/chemistry/chemistry17/chemistry17.html

From that web site:
To explain this phenomenon, we might first break down its name and look at the meaning of its pieces. The first, chemi, means that it has to do with chemicals, and the second, luminescence, that it gives off light. Put together then, chemiluminescence means giving off light via a chemical reaction. To fully understand this definition, though, it is useful to back up and ask what causes the luminescence with which we are most familiar: the light from a lightbulb. In an incandescent lightbulb, an electric current is passed through a filament, or thin metal wire. Because there is some resistance to the current flow, the filament gets quite hot, causing the metal's electrons to become "excited," or enter a higher energy state. When the electrons relax to their normal, or ground, state, they release this excess energy in the form of light. But in this particular process, the metal remains a metal; it does not undergo a chemical change.
A chemical change, on the other hand, occurs when a molecule's bonds are actually altered. For example, the reaction between hydrogen (H2) and oxygen (O2) to form water (H2O) is an example of a chemical change, because the H-H bond in H2 and the O-O bond in O2 are broken when new H-O bonds are formed to make H2O. For the most part, when chemicals undergo change in this way, the reactions either give off (exothermic) or absorb(endothermic) heat. The H2 plus O2 reaction is exothermic. That said, there are a few very intriguing kinds of chemical reactions in which the energy produced is given off not as heat but as light. These reactions are what we term chemiluminescent, or in living organisms, bioluminescent. The most familiar terrestrial example of this "cold light" takes place in the common firefly. In the firefly, an enzyme called luciferase (a name meaning "light-bearing") triggers a reaction that produces energy emitted as light--the flashing beacon from the insect's lower abdomen. Chemiluminescence is also found in some fungi and earthworms. It is most common, however, in the oceans, where many organisms, from fish to worms living at great depths, have glowing organs. Chemists have exploited these light-emitting reactions as markers in a large number of laboratory and clinical tests. The same reaction produces the light from emergency "light sticks" sold to campers and the glowing necklaces seen at concerts and sporting events.

Also, check out http://lifesci.ucsb.edu/~biolum/
This entire web site is devoted to discussions of bioluminescence (and some on chemilumin), but has lots of great pics and simple explanations.

The common glow sticks use a subtance that emits light when it is oxidized by oxygen. The substance is called luminol [(3-aminophthalhydrazide) (C8H7O2N3)] and the oxidant normally used is hydrogen peroxide.

In the course of a chemical reaction, there are always significant changes in the energy content of the reactants as they transform into products. This energy must be absorbed from or dispersed to the surroundings of the system. (Remember the First Law of Thermodynamics, no energy is either created or destroyed). The energy stored in the reactants sometimes is liberated as heat (exothermic reactions) and sometimes also as other forms of energy, in the case of the glowing stick reaction, as light. The process is called chemiluminescence.

I hope this is easy to understand, a graph showing the enery states of the reactants and products and the emission of a photon when the product, originally produced in an excited state relaxes back to its ground state, would help put your message across.

Start with the general way that luminescence happens. An electron has to be in a high-energy state and have a vacant spot that it can drop to while it releases its energy as light. So, lightsticks have compounds that react to produce intermediates with the right configuration of electrons to undergo luminescence. I can't recall the specific reactions at the moment, however as a side note, the patent on the process ran out this year.

Basically, as I see it, light can be "generated" in four ways: from thermal energy (stars, a tungsten light bulb, fire), from reflection (the moon), from absorption and reemission (fluorescence) and from chemical reactions (bioluminescence, chemiluminescence, phosphorescence). A glow stick produces light from an inorganic chemical reaction (chemiluninescence). Bioluminescence, such as you would see in a firefly, is very similar but involves organic molecules produced by living organisms. In either case, light is generated when an electron from a molecule is excited from its normal energy state to a higher energy state via a chemical reaction (oxidation) which requires energy to proceed. As the electron returns to its normal energy state, it releases the chemical energy it absorbed as light. In order to understand this concept, some
knowledge of the visual stucture of an atom (protons, electrons and valence states) and of basic quantum mechanics is needed. I don't think this is at all beyond an 8th grader, but I would suggest using a diagram in an intro college chemistry book as a guide.

In chemiluminescence, the wavelength of the emitted light is proportional to the amount of energy absorbed by the electron (its excitation state). Individual elements or molecules can have several different excitation states (electrons can jump 1, 2, 3 levels, etc., depending on energy absorbed), and an electron returning or relaxing from each of these different excitation states gives off it's own particular color. Since the kinds and amounts of possible excitation states are predetermined from atomic structure (valence states), the overall color emitted by chemiluminescence is specific to the element or compound. Neon gas, when excited, glows red; mercury gas glows bluish; and sodium vapor glows yellow-orange. This is why street lamps are different colors: sodium-vapor street lamps have a particular pink glow and mercury-vapor lamps have a blue-white glow.

Normal light bulbs rely on the use of heat to generate light, whereas fluorescent and neon lamps work on the totally different principle of chemiluminescence, and are more efficient. This might be a good thing to point out.

*For more info on bioluminescence, see: http://lifesci.ucsb.edu/~biolum
*For instructions on how to make your own "glow stick" (for god's sake be carefull!!!!), see:
http://www.geocities.com/ResearchTriangle/2139/vaxessay.html. (If you
need the chemicals, I or someone at UCSB might be able to help you out.)
*For information on quantum states, see: http://www.colorado.edu/physics/2000/quantumzone/index.html

Finally, I have a question! If you discharge static electricity in the dark, you can see a blue spark. (Try rubbing polypropylene socks or underwear together right out of the dryer.) What sort of mechanism do you think is generating the light? Is it the same for lightening?