If they're old enough to understand the story, which is about the struggle to end the slave trade in the British empire, I highly recommend that you watch this movie with your children. It is involving, entertaining, inspiring, informative, carefully produced, historically faithful, thought provoking, well made, and well acted. It's also very prolife, prochristian, and profamily. A rare gem.
Our family (ages 9 to 55) saw Amazing Grace at a movie theater when it was released. All of us liked it, and all of us, I think, are better for it. This is now on my very short list of films that I try to make sure is seen by every young person in my life, along with A Man for All Seasons, Chariots of Fire, and The Miracle Maker.