I would also check their ears for ear mites. I'm not saying that is what is crawling all over them, but just check to make sure nothing is in there ears too.
Fleas. If I were you I'd get advantage for the kitten. Oh no wait...Wait till the kitten is 8-12 weeks old then get advantage or some kind of flea control.
If you're wary of strange poisonous chemicals there are some awesome natural flea solutions at http://eartheasy.com/live_natpest_control.htm#d There are other remedies for all sorts of pests. I think this is an awesome site.
Could very well be fleas - happened to our kittens. Buy a special flea comb from a pet shop or vets to use until the kittens are old enough to use chemical flea control. The comb has tines close enough to catch the fleas so you can kill them. Treat the Mum cat as well! Consider getting a vet to check the kittens out. They'd rather be asked questions that let an animal suffer.
Your kitten is too young for over the counter treatments, Dawn or joy (the dishsoap) kills fleas and isn't harmful to your kitten, but be sure to keep her warm, they can't keep their tempurates until they are alittle older!