A really interesting question to ask. I have actually thought about that day periodically since it happened. I was in fourth grade (Australia) and essentially had no knowledge of, or interest in world affairs. We had class as normal that day until around 10:30 when my teacher received a note which I assume gave some brief information. The lesson was abruptly stopped and we all traipsed across the hallway into a tiny room with a television. We sat there for the next age it seemed (in reality it was probably only an hour or two) watching the various live news reports as they streamed. I can remember seeing the exact same footage of the incident and the same news reports dozens of times.
I can remember my ten year old brain being highly confused about the whole course of events. Intellectually I recognised the destruction of the towers as bad and wrong let alone downright frightening, however it stuck out to me as just so bizarre.