Well, we could bump Adeline's age down, or Cad's up. If you want, we could make Adeline Cad's mother or just a friend who watched Cad's mother turn her into a vampire. And we don't have to make Cad an original vampire, it could be someone older that she knows because I am not set on Adeline being the one who changed her children into vampires.
I am not spoiling anything, I promise. Just that they were made by their mother to protect them from werewolves, which I believe was mentioned in the Vampire Diaries. But I will not spoil any more ^.^