Another variation is to ask the child to draw. First make some sticks, circles, squares, or other shapes on a page, and encircle them. Make for the child a big "bubble" to draw in, and ask the child to draw either the same amount, one more, or one less. Also have your child practice writing numbers on paper.
When you see a fraction, you see a numerator and a denominator. To find the common factor, see whichever number among these is least: The numerator, the denominator or the difference between numerator and denominator.