It depends on how fit/healthy they are.
As kittens it wouldn't take as long to kill them with CO2 compered to using it on a Adult Cat as they have stronger lungs and can inhale CO2 for a longer period of time then other animals, same with dogs.
As kittens it wouldn't take as long to kill them with CO2 compered to using it on a Adult Cat as they have stronger lungs and can inhale CO2 for a longer period of time then other animals, same with dogs.