It’s usually the second exposure that causes the reaction.
There are plenty of anecdotes of close calls. EMTs carry epipens for this reason. And occasionally it’s tragic when one is not administered in time.
But ingestion is not the only way to learn that you have a severe allergy. Skin contact with the allergen with usually result in bad hives. When this happens with a child, it’s scary and tends to result in an appointment with an allergist, who can assess the severity with skin tests and blood draws.
I had an epipen on-hand when my child ate hummus the second time, and started going into anaphylactic shock. They had had a bad hive reaction to spilled milk, so we had already seen the allergist for milk allergies, which were severe enough to warrant an epipen.
It was a terrifying event, and I am very thankful that we had the epipen and that my child did not have a second wave reaction.