Babies are sinners, from the moment they’re alive. Babies are sinners because they’re our babies, and we’re sinners.
“Surely I have been a sinner from birth, sinful from the time my mother conceived me” (Psalm 51:5).
I would agree that we inherit “original sin” as part of our fallen human nature, but that is not something we do ourselves. Can a baby commit an actual sin? Can a baby break one of the ten commandments?