Maybe the person thought "I've given Tylenol to babies in the past and it was fine" without realizing the difference between Tylenol and Tylenol-3.
It's possible the person even thought "Tylenol-3, hmm, that must be another term for children's Tylenol, Tylenol for 3-year-olds. I've given that to babies before."
It's possible the person even thought "Tylenol-3, hmm, that must be another term for children's Tylenol, Tylenol for 3-year-olds. I've given that to babies before."