In order to keep kids from feeling overwhelmed, it's important they stick to a topic that's simple and straightforward. You can smooth the way by helping to brainstorm ideas and making sure that the final choice is something your child seems genuinely curious about. How a tooth decays. How plants reproduce. Parts of a hen's egg.
Plants grow toward light.
Ecology of our school grounds. Fire must have air to burn. How a doorbell works. Causes of the seasons. Using a compass.