Well E.,
this is what I would do if it were my kids who witnessed such a tragic, and disturbing scene like that. If I saw that the "after affects" of that action still affected them in some way, I would set them down together, and explain to them that some people in this life think that there are no solutions to the problems they face, so they feel like they don't want to live. Tell them that this world is full of people without hope, and without the peace of God so much, that they are so tormented in their minds enough, they feel like they don't want to live anymore. Tell them that it isn't the right thing to do, and that isn't the answer to any problem or situation. Tell them, you are sorry for them seeing something like that, and that if they have any questions about it, just to ask you. If you are straightforward about that incident, and take charge, then they will feel like they can come to you if they have a question about it or whatever. I know that it's hard being a parent, but also the sad thing about life is, we won't be able to shield our children from all the bad things in life happening to them, or all the bad things in life somehow or another being seen by them. We live in a world now, that some people don't care what they do, how they do it, or even where they do a list of horrible things. Just tell them if you want, that as long as we are on this earth, we will witness bad things at times, but also tell them that it's good to talk about a certain situation or "accident" if it bothers us. Just take charge as their parent, and try to talk to them about it in a way that they will understand. That is all we can do for our children. I hope this helps!