The Kids Science Experiments website suggests a simple clown craft to demonstrate how gravity pulls an object down. Cut a ping-pong ball in half and fit a paper tube inside half the ball. Tape the tube to the ball so that it stays put, and draw a clown face on the tube. Show the children that the tube will not stand up on its ping-pong base. Take some modeling clay and press it into the base of the ping pong ball. Now the clown won't stay down, because gravity pulls on the modeling clay in the base, making the clown stand upright.
Allow children to use educational websites, like the Project Shum website, to demonstrate that the force of gravity doesn't have the same effect everywhere. Encourage the children to enter their Earth weight into the calculator to see how light, or heavy, the varying gravitational pull would make them on the moon, on Jupiter or on Pluto.
Take the children outside to recreate Galileo Galilei's famous gravity experiment. Climb on top of a ladder and drop two balls of the same size, but of different mass. Drop them from the same height, at the same time. Tell the children to observe and write down which one hits the ground first. This project demonstrates the principle that objects fall at a rate that is independent of their mass.
Sometimes forces other than gravity can affect the way that gravity works, or even briefly offset gravity. The Science Kids website suggests a simple experiment that demonstrates the effect of air pressure relative to gravity. Have a child fill a glass of water up to the top. Tell the child to place a piece of cardboard over the top, taking care not to let air bubbles into the glass as he holds the cardboard. Have the child to turn the glass upside down -- over a sink! -- and take his hand away that was holding the cardboard. If there is no air inside the glass, the greater air pressure outside the glass will press against the cardboard and prevent gravity from making it fall.