We also use diatomaceous earth in our feed. When I worked at the historic grist mill DE was used religiously in any stored grain. It is not poisonous, it does not cause any kind of reaction in people, and is routinely used by many places to help keep some of the weevils from developing in wheat. Had talked to a couple of other mill owners/operators over the years and they say it is standard practice to use it. It is also used in large grain elevator storage.
If it had bugs from the feed store, I would also return it. I don't think that little bit will have hurt the calves.