Cats that have never been to a vet for any kind of check-up or vaccinations....are likely NOT in good health. Therefore, their kittens will LIKELY not be healthy either. They may SEEM and LOOK healthy...but cats do NOT show sickness, unless they are pretty much knocking on death door.
I'll assume you are willing to care for a kitten, knowing the risk and possible cost.
Leave the kitten with his/her mother for 10 weeks. 12 weeks would be better.
When you adopt the kitten, take it to the vet RIGHT AWAY. Expect the first visit to be a few hundred dollars for tests, vaccinations, exam, etc. It is what RESPONSIBLE pet owners do.
Are we good so far?