I, GP, am always amazed that people do not know the functions that their food items (& nutrients within those items) play. That said, many adults don't even know what nutrients are derived from foods they're eating, despite most being clearly labeled. Below is a list of our key nutrients and the main function that they perform. Click on each for more in-depth information.
Protein – repair & growth of tissue
Carbohydrate – primary energy source
Fat – secondary energy source
Water – transportation of other nutrients
Something to note: If you're a primary school teacher teaching Key Stages 1 & 2 - Unit: Keeping Healthy, then I've performed several very basic nutrition talks in the past few years (tying in with that year's curriculum) which have been very well received. For some of these talks, I prepared interactive powerpoint presentations that you're welcome to use. Please contact me for details.