Make sure that the puppies are not dehydrated by pinching the back of their necks. If it "tents" then they are dehydrated and need to be supplemented. If the skin pops right back into place, I wouldn't worry too much. Not knowing how old the puppies are, how many puppies or what breed/size doesn't help here, but the mother may not be producing enough milk for her pups. This is relatively common in first-time mothers.
Do you have some Esbilac (puppy formula) and a nursing bottle on hand in case you need to supplement? Depending on how small the puppies are, you may be able to use a baby bottle (straight nipple). I found that the best bottles are the ones for newborns or premies (people).
BTW, unless there is something seriously wrong, I would avoid actually taking them to the vet right now because they don't have the immunities. If you DO take them to the vet, I would recommend keeping them in a covered box and don't let anyone but the vet see them, and make sure he washes his hands very well before touching the pups. This is for the puppies protection. There are a lot of "bugs" that go through the vet's office.