The language doesn't ruin it for most people. Normal Americans regularly both hear cussing, and occasionally partake in it. Because that's true, your kids are just simply going to hear it one day... And become normal Americans because of it.
I have seen the documentary and honestly, how young a child you even let watch it? It would definitely depend on the child but I really think that the child should be at least 12 or so. I think even if I had a child that age, he would have trouble just watching my reaction to the documentary. I went from sobbing to rage to profound sorrow back to rage. I was exhausted by the end, (and still in a rage).