I think it is important that kids are told about it in an age appropriate way and are able to ask questions.
I was at school in the 70's and 80's and there were a lot of tensions in the world then. Kids at school obviously picked up some bits from the news - which I didn't watch - and I remember one day some kid put it round the class that there was a bomb in the school that was going to go off that afternoon.
I went home for lunch and was walking home with an older girl from school who, during the walk home, explained to me in detail what would happen with this bomb... except she clearly thought all bombs were nukes because she essentially described the fall out from a nuclear bomb - and basically informed me that if I didn't die in the blast, I would die from the radiation afterwards anyway - as would everyone in the area. Bear in mind I was about 6/7 years old at this point and it didn't exactly occur to me that if it was common knowledge that a bomb (of any kind) was actually scheduled to go off in a school that afternoon, all hell would've broken loose and we wouldn't be casually strolling home for lunch!
By the time I got home for lunch, I was consumed with fear. My mum wasn't the type to call the school about things - she was very much the "don't be so silly - get to school!" type. However, when it came to going back to school after lunch I was literally hysterical - crying and asking my mum why she was sending me to school to die! My mum called the head at the school absolutely fuming that some kid was telling everyone they were going to get blown up! Of course it had just been a rumour spread by a couple of kids - but it was only after I'd had proper reassurance from the head teacher and my parents - that I calmed down.
The point is, if you and/or the school don't explain this stuff, kids will come out with all kinds and your child could be sitting there terrified based on some random comment from another kid. While the reality of the situation is very unnerving and scary, it is better for kids to have things explained properly and for them to be given reassurances rather than leave them to their own imaginations, likely fuelled by other kids talking.