How do you tell your kids about 9/11

Last night we told Josiah (our 4 1/2 year old son) about 9/11. We did show him a few pics, but were very careful. No video. After I told him what happened, I asked if he had any questions. He looks at me and very seriously asked why Iron Man wasn't there to stop them. It really puzzled him. To him, evil like that only happens in comics & how could anything that horrible happen in real life? It killed me, I saw a slice of innocence drift away. I was able to explain that real-life heroes ran into the buildings & that they sacrificed their lives to save others.  From there we explained that what the firemen, rescue workers, police officers and average civilians sacrificed is the biggest way you can show someone, even total strangers, you love them. 

"12 "This is my commandment, that you love one another as I have loved you.
13 Greater love has no one than this, that someone lay down his life for his friends." -John 15

We were able to share our fears, our experiences, and our hopes about that day. It was history and all those lives being passed on in memory. We told him we visited Ground Zero (as well as the Pentagon) on our honeymoon. We told him what an amazing place it was. I told him about a moment that healed my heart: we ate at a pizza place across the street from Ground Zero. Their napkins, menus and windows had the same phrase printed on them:
"WE ARE OPEN AGAIN AFTER 9/11." 
My first slice of New York pie was more than a tourist attraction.
It was triumph. 
Some might question our telling our small child about that day. In our parenting I want my kids to hear about something difficult from us.Not TV, not friends, not at school. I didn't want him to see images of that day and have it scare him. I was afraid that with the anniversary upon us it would be difficult to avoid the story and pictures.
Now Josiah not only knows the story of that day, but he knows that he can't live his life in fear & that people can either do what's right or what's wrong. He also knows that God can bring good out of the biggest evil. 
He can live the rest of his life in the shadow of 9/11 (he has no choice, it's history & it changes everything) and make his life matter and live in defiance of that fear they wanted to instill in us. 
Telling my child is keeping memories alive & healing from that day.