Building jet propulsion science projects is a great way for children to have fun and learn at the same time. They will learn the laws of physics when you explain to them that for every action there is an equal and opposite reaction. These projects are also good rainy day activities, keeping young minds active instead of being bored.
To make a balloon jet, you need a balloon, drinking straw and small rubber band. Cut the straw about three inches long. Place the straw inside the balloon opening about halfway, then wrap the rubber band around the balloon neck to secure the straw. Inflate the balloon, then let go. The air escaping from the balloon is just enough for a slow flight, demonstrating jet propulsion. If you have a flexible straw, you can direct the airflow by bending the straw.
Tape a balloon jet nozzle to the roof of a toy car. Inflate the balloon, then let the car go on a hard floor. By letting the car be jet-powered, you demonstrate to the child that there are practical applications of jet power, such as powering a car.
The National Aeronautics and Space Administration (NASA) offers an interesting experiment for children---building a fizzing tablet rocket. This project is a bit more involved, requiring some pre-collected components, such as a plastic film vial and some fizzing antacid tablets. The rocket works by expanding gases. When the tablets hits the water in the vial, gas is created and the lid pops off, inducing thrust. Because this project expels water, it is best done in a sink or bathtub or outdoors.