First of all,
I lost a cat to FeLV that was not born with it and was fully vaccinated against it. Vaccinations are not 100% effective and in fact, if you read enough literature on it, about 10% of vaccinated cats can contract the disease if sufficiently exposed to the virus. My FeLV boy had been living with a FeLV positive kitten for over 6 months before he became ill (long story on how a FeLV+ kitten made it past our screening tests).
If a kitten is born with the disease, vaccinations will not help. Some kittens successfully fight off the disease (usually in the first few months of life) but most are unable to do so. If the breeder didn't test the mom, she might have given the vaccination without knowing it wouldn't help.
You didn't mention the test that you ran, but if all your vet ran was an Elisa test in his/her office, suggest that you bring her back and ask for an IFA test. The Elisa test is only a screening test for exposure to the virus and does not test for the presence of the actual disease in their bloodstream. Only an IFA test can conclusively test for the disease. It takes a full vial that is typically sent to a lab. Most vest office don't have the equipment to run that test.
FeLV is a strange disease. Cats are exposed to it everyday and some will successfully fight it off. Others do not. Life expectancy for kittens born with it is on the average under 2 years, but you never know. 95% of kittens born to a FeLV+ mom will contract the full disease. Adult cats that contract it have been known to live long lives, but others succumb quickly.
You can keep them relatively healthy if you learn all you can with the disease and give them the best food, a regime of the proper medication, and keep them stress free. Someone posted about a FeLV support group. That would be a great start to learn more about it (once you have the IFA test run).
And for the sake of others, if you baby is truly FeLV+, you need to talk to the breeder and ask that her queen be tested. If she is, it will only bring more heartbreak to others if she continues to produce sick kittens.
Please keep us posted on what you find out!