Hello Kat,
 
I believe that your five and seven year old children will be asking about 9/11 during various stages of their maturation so I agree that you may wish to be proactive and start a "conversation" now.  Unfortunately, the events of 9/11 still impact the daily lives of many and your children may already know some information, although it may be inaccurate.  For now, I would suggest that your conversations/answers focus on the historical aspects and how many people are sad on this date for a variety of reasons.  However, it would be important to also highlight that good trumps evil as many people banded together to help others and start organizations to provide assistance.  I would emphasize that children also were part of many grassroots efforts and that they were empowered by helping others. (For example, many children wrote encouraging letters to first responders.)
 
Also, if you live close to where the events happened you may wish to provide a bit more detail.  For instance, families who reside in NYC may wish to show them the current state of the World Trade Center and the memorial.  It has been set up to provide information as well as comfort in a presentation that is sensitive and not scary.
 
Unfortunately, our country has been faced with many tragedies and acts of terrorism in recent years.  The National Association of School Psychologists have placed together on their website information for parents to help children through crises, including how to answer these sensitive questions. Also, feel free to peruse Education.com for answers regarding approaching sensitive topics with children.  
 
Good luck!
 
Louise Masin Sattler, NCSP
Nationally Certified School Psychologist
Owner of Signing Families
http://www.SigningFamilies.com

Dear Kat,
 
As I write this, the anniversary for September 11th has passed, so I imagine that the urgency surrounding your question has subsided somewhat. I like the idea of being proactive and making an effort to be the person that your son hears from first about the tragic events of September 11th, but I'm not sure that he needs to hear about it just yet. I recommend bringing the topic up in context and, for good or for bad, September 11th comes up in conversation most often around the anniversary. Thus, it might be best to prepare for next year.
 
When you do bring it up in conversation, I would start somewhat like this: "This time of year reminds me of something that happened 9 years ago, and I was wondering if you have heard anything about the events of September 11th?" Wait for his response and go from there. If he hasn't heard anything about it, you can explain that there are certain things that happen in history that affect a lot of people, that everyone hears about, whether they personally know someone in the situation or not.
 
You can let him know that some "bad guys", with very different ideas about what is right and wrong, came to the United States from another country and hurt many people on September 11th.  In my opinion,  you don't need to go into detail about planes, buildings, and fields (at his age). But, you can emphasize that the President and others in charge took this very seriously and have done everything within their power to keep us safe. Most people feel that we are safer now than we were then. I think emphasizing our safety will help relieve any anxiety he may experience.
 
I also included a link to an article below from the NYU Child Study center about September 11th. You may find that of use, as well.
 
I'm sure others will have good advice for you. Go with your instinct and use whatever language seems most appropriate given your son's maturity.
 
L. Compian, Ph.D.
Child Psychologist
Education.com