Perhaps you could try comparing the way they are raising to dog to how they are raising their baby - would they toilet train their baby by yelling and smearing the dirty nappy in it's face every time they change it? Will they be using a cot which the baby can't climb out of (after all, the crate is really just a cot for dogs!)
Obviously dogs and babies are different and can't be raised exactly the same, but this approach may be enough to make them think.
I am anticipating lots of negative comments from family about "cages" when I crate train my new puppy - I'm thinking of making a wall hanging saying "puppy's room" to get them to see it a different way!
