The best way for me to do this type of thing is to understand the basics, understand the fundamentals. How does something work, how was it made and why does it work that way - it's a tough engineering question, but it doesn't have to be. There are two common ways to understand a problem like this: reverse engineer it (take it apart) or make it (follow the instructions). Most inventions are just deviations of other ideas, we all build upon each others ideas to create something new.
We have to give children these building blocks, and the easiest way might be through cooking. Find their favorite food dish, and then make it with them. Explain how and why it is made the way it is, and ask them if there is anything they would do to make it better - how can they improve it? Believe me, it will get their mind thinking a mile a minute and the results will surprise you.