Hopefully you are not dealing with something like Parvo. Take the puppies to the vet!
Has the puppy always had this problem or is this recent? How much blood? Agree you should probably take it in to get checked.
Is the blood in the stool itself or around the anus or both? Best way to find out is to take a stool sample in to your vet and find out. If it's not in the actual stool, a physical examination by your vet would be in order.