I'm afraid I'd be seeing a vet about that.

What type of kitten?
Longhaired kittens open their eyes later, so if you have moggies, you may
have some short hair and some long hair.

How long has only one eye been open? If less than a day or two, wait. It
takes a little time. Mama may need to clean the kitten's eyes more.

[I cannot over-emphasize how gentle you need to be in the following]

Warm water is to remove any goo that has built up in the eyelid, much like we
can wake up with "sleep" in our eyes, gumming them up. Don't put anything in
the water. If you think the eye should be open, GENTLY palpate the eye socket
to make sure there even is an eye present. Deformities happen. If there IS
one, soak a clean, white cloth (you don't want any chemicals or dyes in the
fabric) in warm, but not hot, water, and lay it gently over the eye until the
cloth cools to room temp. Check the eye (don't have a bright light on the
kitten, which would be uncomfortable to a newly opened eye). Is it gummy? If
so, gently rub the cloth over it and see if the goo comes away and GENTLY put
a little pressure on the lid to encourage it to open. Nothing? Try once or
twice more. Still nothing? Vet time.
If at any point the kitten acts like it is in discomfort from you fussing
with the eye, leave off and make a vet appointment. Not only might it save a
lot of trouble having an expert handle it, it's really better that your
kitten, so new to the world and gaining all the experience it can, not
associate you, the most important being in the world after its Ma and sibs,
with any pain at all.

HTH

Kim. my cat just got sick and was wondering how you made out with your kitten's
eyes.   Don