those things are huge, ive stood right beside one before (a foot away on the other side of the fence!!) and couldent believe the size. Its good that your kids were able to see them.
if you think the plains bisons are big, go up across the McKenzie river and see the wood buffalo. They make the plains bison seem tiny.
These guys were big enough for me. As he was walking down the middle of the road and I was behind him in the car.......wow I hope he dose'nt turn around.