This is an important question! It is also important to teach this topic when children are young. Here are a few suggestions.
Frist, you might want to watch a movie that kids enjoy. In this way you can capture their interest. Veggies Tales are a great start. After the movie you can discuss that they learned.
Second, there is no better teacher than example. So, as an adult you will have to model it consistently. The key word there is "consistently." If they see compassion on a regular basis, it will be their best teacher.
Finally, you might want to take them to some community service project. My sons's school has these several times a year and they learn about loving and caring for those in need.