Distinguish between the numerator and the denominator. For example, the numerator of the fraction 1/2 is 1 while the denominator is 2.
Find the common denominator between the two fractions by multiplying the denominator until they are equal. Make sure your fourth grader knows if she multiplies a number to find a common denominator, she must multiply the same number by the numerator. For example, if you are adding 1/2 and 1/4, you will need to multiply the 2 by 2 in order to get 4. Since you multiplied the denominator by 2, multiply the numerator of the 1/2 by 2 as well to get the fraction 2/4.
Add together the fractions. Make sure your fourth grader knows to keep the denominator constant and only add the numerator. For example, 2/4 + 1/4 = 3/4 (not 3/8).
Simplify the fractions if applicable. If the numerator and denominator can be divided by a common number, they can be simplified. For example 6/4 has a numerator and denominator that can be divided by 2. After dividing the top and bottom by 2, you get 3/2, which is the most simplified form.