Baby calves can really suffer in the heat...especially oppressive heat. And, should they scour, they can die from dehydration super fast.
When I was raising baby calves I kept them shaded. I had pens and hutches set up under every available tree. I also misted them down with water and gave them electrolytes if they even looked at me funny.
I don't know if you have any way of misting them or giving them electrolytes periodically, but if you do, it would be worth the effort.
Alice