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.
This affords me the opportunity to publicize World Maths Day which took place on the first Wednesday in March. There students of all ages from all over the world went online (it's free) to play mental math against each other. Last year 287ꯠ students from 98 countries participated to answer over 38 million questions. This year they hoped to break 50 million questions and did so magnificently answering over 150 million questions and I am sure they had great fun doing it.