I wouldn't get into the actual chemical reactions and the electron transport chain and how chlorophyl bounces electrons around.
Something along the lines of, "The plant uses energy from the sunlight to convert carbon dioxide and water into sugars and oxygen gas. This is a chemical reaction that uses energy, and the energy is effectively stored in the chemical bonds of the molecules of sugar that it makes. You could talk about how a plant can go on to string sugars together to make starch or cellulose, if you like.
Later, the plant or an animal that eats the plant can get the energy back out of the sugars by reversing the process that the plant went through to make them. Instead of using up energy and giving off oxygen, disassembling sugar into water and carbon dioxide gives off energy that the cell can harness to fuel its biological processes, and requires oxygen.
It might be a good idea to have a very simple diagram of the input and output of each process. Maybe it would be good to talk about how sugar is made out of carbon, oxygen, and hydrogen.
If you want to talk about ATP, you could talk about how using a molecule of glucose to make several molecules of ATP is sort of like going to a change machine with a ten dollar bill and getting quarters that you can then use to buy snacks from a vending machine, run the machines at the laundromat, use a pay phone, or play arcade games. (Things that won't usually accept a ten. Although I realize there are high tech exceptions to the rule.)
Edited to add: If I were you, I wouldn't explain the electron carriers. It's not that an 8 yr old is incapable of comprehending them if they wanted to, but that stuff can put a class of university students to sleep. My personal opinion is that if you try to explain it in that much detail, they will all zone out. Unless you have explicit instructions that you have to cover the details, go way more general.