How to explain it to a child depends a bit on the child, but for the most part I've found that with my kids the things I'm think will be scary to them are usually not the things that truly worry them. Another important thing to keep in mind is that your children may be talking about it at school - in detail, if not in class they may hear about it from friends. If my children were asking for more info, I'd give it to them - I'd for sure tell them about the airplanes flying into the buildings because they *are* going to hear about that.
FWIW mine have known about the airplanes etc from the time they were in first grade, possibly earlier. They've all happily climbed on airplanes many many times since then and don't have second thoughts about it - the act of terrorists several years ago is far removed in their minds from flying to see cousins or on a fun trip.
Best wishes,
polarbear
