I am bemused by the number of posters telling Salsmum she must keep silent about her views. Is this really the way your families work, not daring to offer even the slightest criticism to relatives doing controversial things?
There's a bit of a difference between telling an adult who is an equal, and a child who is impressionable. She is welcome to tell the mother and father.
But by making her disapproval known to the child, she's in essence asking a child to pick between his parents and her approval. That's what's going to alienate her. If her GC is feeling cruddy every time he talks to her about a hunt he's proud of, he'll stop talking to her, or his parents will stop bringing him because he goes home feeling bad.
It's the same reason you don't tell an athiest's child about your religion, heaven and hell, and all that. Even if you truly believe they need to be saved.
You let the child live according to his parent's morals and principals until he's old enough to start deciding for himself.
So, 7 years old is not the right time to express disapproval outside of a "I don't personally like hunting, but I'm so glad you had a good time". 13, might be a good time to talk about the ethics of it because they are at the age to decide for themselves what's right or wrong.