The steps for subtracting fractions is well illustrated in the fraction calculator at the link below. That page has a more detailed discussion of common denominators, mixed fractions and other topics that may help understand the steps necessary to subtract fractions.
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.