There's video of a five-year-old named Sadie on YouTube, and her parents started recording after she found out her baby brother won't STAY a baby . . . and she was DEVASTATED.
Apparently they also explained DEATH to her for the first time, because she freaks out about that too. (Search for "Sadie Doesn't Want Her Brother to Grow Up." Check out the look the baby gives her at :31.)