The experiment asks the question: Do objects with different masses fall at the same speed? Before Galileo did his experiment, it was thought that heavier objects fell faster because of their greater mass. Galileo tested this hypothesis by dropping a 1-pound and a 10-pound ball from the top of the Leaning Tower of Pisa, but a ladder will suffice if you want to recreate the experiment at home.
The experimental materials are simple to collect -- find a stepladder, two balls of the same size, but different masses, and a video camera to you want watch the instant replay. Cell phone video will do if you don't have a video camera.
Before you begin, talk to your children about some basic concepts, including who Galileo was, and what gravity, mass, and acceleration are. Discuss data collection, because every good scientist takes more than one set of data to verify an experiment, and doing the experiment multiple times shows that its results are reliable, measurable, and repeatable.
Gather the materials and assign roles. Depending on the children's ages, they can drop the balls, collect data, or operate the camera. If they are too young, you should drop the balls from the step ladder. Make certain the children who are not dropping the balls are a safe distance when they fall, for their safety and for the protection of the camera equipment. Run five to 10 trials to establish the reliability of your results.
Once the data has been collected, and the experiment repeated for reliability, review your results. In the video, is there a difference in time when the balls hit the ground? Ask the students why they think the balls are hitting the ground at the same time. What force is in effect here? Why is gravity more important than mass in acceleration? The answer is that the Earth's mass and gravity far outweigh that of the balls, so it is what is influencing the results.