Cats often have upper respiratory infections. Because you are seeing green/yellow gunk in the eyes, you need to see the vet for antibiotics so that the cat can recover.
You can wash the eyes with warm saline to remove any gunk in the eye until you can see the vet. There are no over the counter meds that are safe for cats.
When you take a litter of kittens to the vet, the vet will usually charge you for one office visit and see the whole litter. If you have doubts about this, ask the vet when you make the appointment.
Let me know if you have more questions.
