When I was in the 6th grade I learned about the the experiments conducted by Galileo. I was truly fascinated and wanted to replicate it. Galileo and many of his contemporaries experimented with falling objects and proved that even objects of different masses will fall towards the Earth at the same velocity when dropped from the same height. However timing and other factors like wind resistance stood in their way to prove it conclusively. They had to used inclined planes and balls of different masses to conduct their experiments. However, Galileo categorically stated that objects in a vacuum (meaning no air), would fall to the Earth with a constant acceleration (which we today call as the acceleration due to gravity). According to him a feather and an Iron ball when dropped from the same height in vacuum will reach the ground exactly at the same time.
As a kid I tried explaining this concept to my friends, but I was not able to effectively convey it. They strongly believed that a piece of paper will fall slowly when compared to an iron ball irrespective of the surroundings. When I took this issue to our science teacher, she too said that a heavier object will fall faster (I studied in a crappy school).
I came across this video today and I wish I had access to this video back then. Most of us probably know that when two objects are dropped in vacuum fall at the same rate, irrespective of their mass. However I am sure that you would have never seen a demonstration of this. It is incredible.
You can also conduct a small experiment at home to teach your kids about this concept: Tennis Ball vs. Paper
You will need a tennis ball and a A4 paper. Go to your terrace, hold the tennis ball and the notebook paper (horizontally) at the same height from the ground. Ask your kid which would strike the ground first if they were dropped simultaneously. He will most likely pick tennis ball, drop both the items and the tennis ball will hit the ground first. Now do it all over again and crumple the paper tightly in to ball. Now ask your kid which would strike the ground first if both were dropped simultaneously. Even now your kid will most likely pick the tennis ball, however when you drop both the objects will hit the ground almost simultaneously.
After the experiment, you can teach your kid about acceleration due to gravity and the constant g = 9.9 m/s2. You can teach them the relationship between distance, time, velocity and acceleration. If your kid is 10 years or older then you can also tech him about momentum.