We get a pack of washcloths from a discount store and use them in the barn for things such as this. We use warm water and wash faces, eyes, udders, or behinds - whatever needs doing. You can use NO MORE TEARS shampoo if you need a mild soap.
I have always found that human eye wash or optrex is great is washed out with it or even put on a cotton wool piece and squeezed over the eye, eye ointments for humans are very good as well, I had a loan horse a while back that got an eye infection from a fly bite and this cleared it up in about 2 days. And made him happier too