In my experience a healthy cat or kitten will produce rather dry poop that doesn't stick to the fur. That suggests that your kitten is having stomach issues and may need a vet visit.
Kittens usually have more sensitive stomachs than adult cats anyway, and from what I've seen a kitten with stomach problems is much more likely to have sloppy, messy poop than to throw up - and sloppy, messy poop will stick to fur. Kittens will also be more likely to step in it afterwards (cleaning up after that is not fun!) while they're trying to cover it up.
What I did with the last longhaired kitten I had when she had issues was to start with a box of tissues and use the tissue to "wash" her butt. You might get everything out this way, or you might need to brush/comb the rest out. I recommend doing this when the kitten is sleepy or you'll be trying to clean a squirming ball of energy that wants to play with you. Don't forget to thoroughly wash your hands and the brush/comb after you've thrown the used tissues in the trash.
If there's a lot of poop to clean, you might need several sessions, and if the kitten is having serious stomach issues, you might want to trim the fur there until you can get the stomach problems settled.
A few other thoughts:
- there's a difference between a healthy, more or less dry poop hanging from the kitten's butt and poopy mess sticking to the fur. The dry dangler is probably either the kitten rushing the job to get back to the more interesting things or a partial hairball that hasn't been fully passed. If it's a hairball, there'll be kitten fur holding the poop and you'll want to give the kitten hairball treatment.
- As others have said, the breeder really shouldn't have given you the kitten so young. You'll want to watch for other health issues.
- If your kitten has ongoing diarrhea, you need to see a vet.