Teach by example. Depending on your age, you possibly grew up in a time when you were taught that there was to value to tolerance or diversity. I live in the south. Back before the Civil Rights movement, there was definitely a class structure. Fortunately for me, my parents taught me to treat people with respect. I try to do that.
All of the suggestions made thus far have been good. Taking children to another country may not be practical for many people.
There is a song in the move South Pacific called, "You Have To Be Taught To Hate."
Unfortunately, that is very true and is practiced by many people today.