The safest option would be in a crate. A quick consult with the great and powerful Googles says that the Acadia is a three row SUV. I'm assuming you have the kids in the second row? If you fold the third row of seats down (or even remove them, if that's an option), the you should be able to get a crate in there. Otherwise, putting the crate in the back row of seating is an option.
The next safest option is to have the dog in a seat belt harness, and strapped into a seat of their own.
The least safest option is the dog loose in the car.
The next safest option is to have the dog in a seat belt harness, and strapped into a seat of their own.
The least safest option is the dog loose in the car.