When my son was three and a half, he was playing with a toy flashlight. I don’t know what he was doing but it happened very quickly. He somehow he took the toy apart, put one of the two batteries in his mouth and swallowed it by accident.
He told me just after it happened! Thank goodness was old enough to know the danger and tell me what he did. I then saw the toy and the other battery.
I called Poison Control to get some advice. While I was on hold, I quickly searched the internet and the consensus was that the battery would likely pass.
Something similar happened to a young girl, but was far more dangerous.