Similar to many replies, I would also introduce my lesson on rockets using the book Aliens In Underpants Save the World. Children love this book and the illustrations are fantastic.
Designing a rocket can be difficult for small children but it can be very enjoyable. I would bring the children outside and show them how a Stomp Rocket works. The children will soon realize that the more force you put in the higher the rocket will go.I would also draw their attention to the fins on the rockets that are responsible for its stability.
A simple rocket can be made using a straw, string and a long balloon. The string is tied to something stable at both ends after the string is passed through the straw. The ballone is inflated and taped to the straw. The balloon can be decorated and fins can be added. When you let go of the balloon it will travel along the string.
Children can be asked to predict what might happen and test this experiment using more of less air.
Videos from NASA and Space X can be shown to the children in order to inspire their creative imaginations.