My daughter was almost 2 when 9/11 happened, and my son was 6 days old, so obviously they were way too little to get it.
My husband is a firefighter here in NJ a few miles out of NYC, there are references to 9/11 everywhere, nto just in parks and stuff, but also on firetrucks that thay see all the time. It's talked about still, so my kids have asked.
I told them very terrible people flew planes into two really tall buildings and the buildings collapsed, killing everyone who couldn't get out as well as police officers who were trying to help them out and 343 firefighters who were doing the same.
I haven't told them about the Pentagon really , just in passing, nor the crash in PA, but they know what it is. Not sure if they would say Oh? 9/11? Sure! It was when the buildings fell.
But if you asked DD especially if she knows that buildings fell in the city and what happened, she knows.