What to use if straining to poop:
One of the reasons there might be straining to poop is that the mother dog usually licks the puppies to stimulate their muscles to poop. You could try using a wet washcloth under the tail to simulate the mother’s action

If constipated (stopped up and not pooping):
–     Can feed canned pumpkin for both diarrhea and constipation. It evens out the intestinal tract, oddly enough. The canned pumpkin is often given just once a day.
–     A little plain yogurt for probiotics would also help
–     Prior to ME, whenever xxxx was constipated we always gave some pure canned pumpkin and it helped move things along