Have the kittens had any dewormer?
Has the mom been dewormed?
Is the kitten strictly nursing the mom for nourishment?
Have you seen how the bowel movement looks?
Dr. Debbie
Most of the time irritation like this is caused form diarrhea. The kittens are just now of age to be dewormed and I would go ahead and get some dewromer containing pyrantel pamoate and dose according to the label. You can do this for the kittens at ages 2, 4, 6 and 8 weeks old until they are weaned.
If you can dose the mom that would be good as she may be the source of the parasites.
The other possibility would be a bacterial or fungal infection. You can put an antibiotic ointment on the area such as neosporin and it won't hurt if the mom licks that a bit.
Let me know if you have more questions,
Veterinarian
Full time practicing companion animal veterinarian.