There are kits now with allergens that you can feed your kid to test for this kind of thing[0]. The idea is that you have them eat it in the parking lot of a hospital and see what happens.
Here's the thing: if there really are kids out there who will drop dead within minutes of eating a sesame seed, surely some of them will discover this the hard way, i.e. by accidentally consuming a sesame seed and dying. But not once have I ever heard a news report about a kid dying this way, and I can't find any data on how many people die this way. I also can't imagine any reliable way that one could possibly learn that your allergy is so severe that a sesame seed will kill you without having at least some people actually die.
All this leads me to suspect that the belief that sesame seeds are potentially deadly in small doses might not be solidly grounded in facts.
Actually, we can estimate odds even with zero deaths.
The thing is regardless of the trigger anaphylaxis is anaphylaxis. The severity differs, the mechanism is the same. We can see the distribution of reactions and estimate the number that will be lethal even if we have no examples.
(And I rather suspect that a fair number of the lethal cases don't get diagnosed. I don't believe autopsy will reveal what set it off unless the contaminant is obvious.)
Here's the thing: if there really are kids out there who will drop dead within minutes of eating a sesame seed, surely some of them will discover this the hard way, i.e. by accidentally consuming a sesame seed and dying. But not once have I ever heard a news report about a kid dying this way, and I can't find any data on how many people die this way. I also can't imagine any reliable way that one could possibly learn that your allergy is so severe that a sesame seed will kill you without having at least some people actually die.
All this leads me to suspect that the belief that sesame seeds are potentially deadly in small doses might not be solidly grounded in facts.