Kitten has diarrhea and anus is swollen?

Please bring the found kitten, also 6 weeks old, to the animal shelter. These little ones desperately need a mother, siblings, or at least companions, as they're just learning about life at that age. You need experience, and many vets quickly become overwhelmed with such small animals because there's not much they can do, and few have kittens as patients very often.

Then baby wipes? I hope something that contains absolutely nothing, because any fragrances etc. can even be harmful to little ones. You normally put lukewarm water on a damp cloth for cleaning, so even with such small things, you can go wrong.

I think it's really great that you want to help the little one, and I'm not concerned about whether a stray animal is being misappropriated. Kittens are a completely different matter. They not only have health issues, but also imprinting phases that only occur at certain times and can't really be made up for later, and they can cause lifelong damage to their social behavior. This whole mess is just too long for someone without cats or experience to raise such a small kitten.

There are people here who no one can help medically. There are a whole host of possible causes for diarrhea, or parasites that won't go away without treatment. Kittens are given a combination of wet food and mother's milk for 6 weeks. If the food isn't very good quality, it might be enough, or not. Monoprotein might be enough if they're sensitive. There are simply too many triggers, and kittens that age are many times more sensitive than when they're older. Their upbringing determines too much how they develop, so please take them to the animal shelter.