When teaching children about how the kidneys function, it is always preferable to simplify the topic in order to make it relevant to them. If they can relate it to their own bodies, there is a much better chance of them retaining the information. Human anatomy can be complicated even for adults, so a good place to start teaching kids about kidney functions would be to show them how the kidneys work in their own bodies -- and to explain how what they drink or ingest affects their kidneys.
Produce a large image of the human body, with the vital organs such as the heart, lungs and kidneys clearly illustrated, in order to show the position of the kidneys within the body. Ensure the vital organs are labelled in large letters to differentiate them and avoid confusion.
Make a life-size model of a kidney, which could be effective in getting your child to relate to the look and feel of the organ. This can be done by moulding polymer clay into the shape of a kidney, then baking it in your home oven at a low temperature. The liquid in the clay evaporates and the particles fuse together, resulting in a kidney-like model.
Use the model (or alternatively a large, separate picture of the kidneys) to show how the kidneys function. Explain how blood becomes polluted with waste as it travels through the body, and that when it passes through the kidneys, they filter out the urea from the blood in order to cleanse it.
Clarify how the kidney passes the waste it has extracted from the blood -- in the form of urea -- into the nearby bladder for storage, and how this is then later expelled from the body as urine. The reference to something tangible (such as an everyday bodily function like urinating) will allow the child to see how kidney functions have a direct impact on his body.
Provide your child with illustrated books, designed specifically for children, on human anatomy. This will ensure he has a ready source of reference about the kidneys and how they function in relation to other vital organs. Later, introduce the concept of the damage that can be caused by smoking tobacco or drinking excessive amounts of alcohol, and the potential harm caused by high blood pressure and diabetes.
Illustrated books for children are the most convenient source of information and will have the added bonus of getting your child interested in using books from an early age.