TINY KITTEN POO
People who are new to fostering always freak out when tiny baby kittens don't poo for a few days. Don't freak out!
As long as they are nursing, it is normal for a kitten to poop every 4-5 days. Only after they start eating solids does the poo output become more regular. What is important is that a nursing baby pees every time he eats--poo is much less significant. Remember that babies can't go to the bathroom by themselves until up to a month old--you need to stimulate them to pee/poo at every feeding.
Tiny baby poo is usually a dark yellowish color when they're on commercial formulas. A kitten who is eating mommy's milk has more normal brown poo. I almost never get a so-called "normal-looking" poo out of one of my little cat butts.
Diarrhea in nursing babies is reason for concern if it continues for more than a day--go to the vet!
Poo that's any weird color also merits a doctor's visit. Weird colors include white, green, red...anything but dark yellow/orange and brown.