The saying "fighting like cats and dogs" is birdlike: It flies out the window when a dog steps in and fosters kittens. Often when kittens are orphaned, a dog instinctively will take over the mothering responsibility until they're old enough to be weaned. It's lifesaving for a grieving dog, too.
Kittens' Nutritional Needs
It's best when the kittens can nurse from their natural mother for at least the first 24 hours of their lives. This is because in the first 24 hours after giving birth the mother cat produces colostrum, a substance that naturally provides antibodies to protect the kittens from infection. Overall, the vitamins and minerals that kittens and puppies require are very much the same, although kittens need more crude protein and less crude fat than puppies do. If it becomes necessary for a foster animal to come to the assistance of nursing kittens, a lactating dog can provide milk that will be similar enough in nutritional value to the milk that the kittens would get from their mother.
Encouraging the Dog to Foster
If you have a dog that is already lactating, putting the kittens in with her litter and having them nurse alongside the puppies is the next best thing to having their own mother. You cannot force a dog to accept babies that aren't her own, so there are no guarantees even with a dog that's nursing puppies. But in many cases the dog's nurturing instinct takes over and she will accept the tiny felines as part of her litter, not only feeding them and cleaning them but protecting them as well. It's not unheard of for a non-lactating dog to start to produce milk for an orphaned nursing animal when the need arises.
Having a dog as a wet nurse for orphaned kittens is a great advantage over having to hand-feed them. They will be able to eat on their schedule instead of yours, and you won't have to worry about whether you're doing it right or if they're getting too little or too much to eat. The mother dog will also take care of keeping the kittens clean and warm, two other things off your "to-do" list. And kittens who are raised with puppies will be well socialized with dogs, which can make it easier to find homes when they are friendly and playful with other species.
When You Should Step In
There are times when you'll have to step in and do the fostering yourself. If your prospective surrogate doesn't readily accept the kittens, they could starve -- plus they can be in danger of harm from the dog. If the dog does accept the kittens but you can see that they are too small to nurse from her or they don't appear to be thriving and gaining weight, you will most likely have to bottle-feed them. Keep a close eye on the situation and contact your vet sooner rather than later to assure that the kittens' health isn't jeopardized. If you end up bottle-feeding the kittens in a case in which the dog accepts them other than nursing them, she may be able to help you by keeping the kittens clean and warm after you feed them.