One great way to teach it is by using math worksheets. One could make worksheets that are fun for children to complete. Adding graphics such as fruit and candy and having the children add them up will open their minds to it and have them learning in a fun way. Also, the sense of accomplishment the child has after completing the math worksheet correctly will encourage him to continue learning the subject.
Teaching the subject to an older child may be a bit more difficult. There are many math problems out there that can be used to bring it down to their level. Using this subject in an everyday situation is a one idea. For example, tell a story that involves that child. If he goes to the store and buys a gallon of milk with a five dollar bill and the milk is only three dollars, how much money will he have left? Using word problems will help the child understand it more.