Well, the responses to this have been very interesting. Thanks all. I especially appreciate the compassionate responses: thank you 
Although I did use the term "essentially developed PTSD" a bit flippantly in a way, I also used it advisedly, in that I'm talking about the specific criteria used to define it. My child's been experiencing the exact constellation of things associated with the label. And I've been in close contact with colleagues who know. To those dismissing the experiences: if you're going to read threads, I'm not sure why you wouldn't listen to the people directly involved (who know the details of the situation) over guesses made on the internet.
And to those commenting on the unusual nature of the response: yes, most kids don't experience trauma reactions to most drills. I agree. But to go to the more judgmental "that's not normal", etc., from there is also strange. Although we know some of the factors associated with sensitivity to these kinds of things, even the world's top scientists don't know with any certainty why some people develop trauma responses to some events and others don't. There are huge numbers of uncontrollable factors, as well as lots of variation we just have no idea about. If you know all the answers, please do go tell these experts, as they'd love to hear them.
I think it's interesting that we tend to say "it's fine, because that's the guidelines", or "it's fine, because it's been going on for years". We know that kids are experiencing pretty bad mental health overall, and that schools can contribute to the sense kids get that the world is a bad place. There's also some research suggesting that anxiety and depression rates increase after lockdown drills. And I'm not sure there's much (any?) data showing that they actually save any lives. So I'm not saying the drills are always bad, but surely we have to question these things sometimes? I think it's pretty safe to say that sometimes, kids will genuinely believe something bad is happening, and some of these kids will then have a trauma reaction. To dismiss it by saying "well, those kids must have had problems anyway" is a) unscientific, and b) inhumane. Just my thoughts.
All the best, and thanks again.