A child is most probably not going to ask "Mommy, why are your eyes wet?" when they see their mother crying. They've cried themselves since the day they were born; they know full well that tears mean distress. They might be confused the first time they encounter someone crying tears of happiness, since a person is showing all the signs of distress and yet there's clearly nothing to be distressed about.
Likewise, a child is not going to see an adult in pain and ask, "Daddy, why are you acting funny?" Aside from the fact that children are freaked out by things that start acting in strange and erratic ways (ESPECIALLY things they've generally come to trust as safe), even the youngest child knows pain and how people act when they're in it- they have, after all, been in pain themselves.