This is wild to think about kids being sheltered from what happened when you were just in 2nd grade yourself when it happened and had to watch the replays over and over and over and over endlessly for years on the news and in school.
This is also not a uniquely 9/11 related issue. Most people shy away from explaining events of tragedy or grief to kids. Then when they are older and find out it is extra shocking because they think it was not as bad as it was. They didnt know any better though.