I grew up on a farm and my father butchered the steers and chickens we raised. There was a no nonsence attitude about the animals destined for the table. First, it was made perfectly clear that they were NOT pets. They were not given cutesy names and treated like pets. They were table food, period. We did have our favorite chickens, not for butcher, that were our pets. We helped with the butchering (to this day I HATE pulling plucking feathers).
Make it clear, from the beginning, that these are food and don't waiver. The first butchering is the worst. Put up with no tears about it and don't give in. Next time is easier. After that it becomes a part of life.