Ah, starting to make more sense now.
You have 5 cockerels and five pullets?
You don't live on a farm (?)
Bad choice of reasoning with your sons then.
Can't be long before your eldest tells you unless the creatures actually live on a farm they're not farm animals.
If either, or both of your sons have become attatched to these chickens and your line of reasoning given for needing has been found wanting then you've got ten pets as far as your boys are concerned.
If you kill any you'll be a pet murderer. Children remember things like this for the rest of their lives.
Just out of curiosity, what did you tell them when you ordered the chickens/when they arrived?