If the mom is caring for them it is very likely they are going and you are not aware. Mother dogs lick and ingest the pups waste so you will not see any traces.
How do their belly's feel? you can give a few drops of mineral oil to each to help them to go if they are constipated, but be sure there are signs and symptoms of constipation first. You dont want to cause diarrhea in a puppy, they dehydrate very quickly.