Sophie Peterman tells the truth by Sarah Weeks

"PreS-Gr 2—In no uncertain terms, a girl warns readers about the perils of a new sibling. Looking like an alien at first, and the object of unwarranted praise and attention, a baby is prone to all manner of gross behaviors. Sophie reveals that the situation doesn't get better as the infant grows into a toddler (known as a "monster"): stealing Halloween candy, swallowing lucky marbles, and exhibiting general uninhibited behavior. She softens, though, when the monster begins to focus affection on her but leaves readers with a warning not to reveal this softness to parents lest they repeat the experience. "
from School Library Journal