Use some very basic experiments to get children to understand the concept of a center of gravity. Have a child try to balance a ruler horizontally on a single, outstretched finger. When he places his finger in middle of the ruler it will balance. Gravity pulls down equally on either side of the ruler. Explain that this is because he has found the center of gravity.
Tie a string in a loop and stick it over the end of a long wooden ruler. Place one end of the ruler on a surface such as a table and hold it in place. Put a hammer's handle through the string so that the head of the hammer is underneath the table. The ruler balances on the edge of the table unaided. This is because the weight of the hammer's head is balancing out the center of gravity of the construction.
An object's center of gravity is the part of the object at which the majority of its weight is concentrated. Find a human's center of gravity in the torso, not the fingertips. Use a ball to illustrate this. Roll the ball and point out that it will land in any position. Stick a small sticker on one side so the children have a reference point. Repeat the test a few times. Attach some clay to one side of the ball and roll it again. Repeat this roll until you have shown that the ball always lands with the clay closest to the ground. The shift in mass causes a shift in center of gravity.
Several fun experiments illustrate the limitations that the human center of gravity imposes on people. Get a child to bend forwards and grab his toes with his hands. Ask him to jump forwards without letting go of his toes. This is impossible because the body is unable to shift its center of gravity before moving. Sit a child in an armless chair with her feet flat to the ground and back against the chair's back support. Ask the child to try to get up without lifting her feet or bending her back.