Make the ideas relatable. When discussing specific machines, invoke ones that they know well. These could include your family car, a see-saw or other items like their bicycles and skateboards. Not only will the child be able to visualize better, but you can reference and access the actual machine, while delivering your explanation.
Mention the mechanical components of simple machines. Some essential ones are the wheel, pulley and lever. Explain how these mechanisms multiply or enhance the power of human beings, thus making the work to be done easier. That is, after all, the very function of any machine.
Introduce the concept of force. BrainPopJr.com suggests an analogy of a ball. When the ball is thrown by the child, they are exerting a force to make the ball move or fly through the air. Alternatively, when they try to catch it, they are exerting a force designed to cease the motion of the ball. This conceptualization can lay some fundamental groundwork for introducing more complicated mechanical ideas.
Conduct a scavenger hunt. Have the kids go around the house and collect safe objects that could be tools. These include -- but are not limited to -- hand tools, pencils, nutcrackers and anything resembling a ramp. Then, have them try and classify what type of tool they are -- such as a lever or pulley. Even have them locate points on the human anatomy that are tools, such as hands, joints, muscles or teeth.