Using two graham crackers, divide one in half horizontally and the other in half vertically. Take one half from each group and ask the child to prove that they are both equal to one half, even though they look different (For example, he could take each half and break them in half again. Then there will be four equal parts. You could use the four parts to create a whole cracker again).
For the younger child, use just one half of the cracker so there is only one line to choose from. Prove that both parts are equal by placing one part on top of the other.