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.
You may want to let them try and work out how long it will take you to travel somewhere when you get in the car, by telling them what speed you'll be traveling and how far away your destination is.