They're old enough to understand what their food is, but I wouldn't give them nightmares.

They don't need to watch earthlings.

But ... They're going to ask questions.

Why aren't you eating bacon any more? Well, because pigs are really smart, very kind creatures and if we don't need to kill them to survive, why should we? And why aren't we having dairy milk? Because the cows that we take milk from are kept in really sad places, they're hurt and they have their babies taken away. We have plenty of other things to drink!

You know your kids. Tell them as much truth as they can handle, and then let them process that truth. Let them know that you trust them to make good choices in life, and that you've made the choice that you won't be having any animals products at your house.

Good luck. You'll make it work.