One of the twilight movies. In a flashback, they show the Volturi massacring a group of vampires in which one of them had converted a child. Being a child when turned made them have no regard for secrecy or discretion, which would have led to them exposing the existence of vampires.

Which is why they were all coming to kill the Cullens when the mortal Bella gave birth to Edwards half vampire offspring, as another vampire saw a young girl with vampiric powers, not understanding that the half vampire girl would age rapidly until adult and then staying adult for eternity.

My summary may be slightly off. I read the books like... 16 years ago? Idk.