Discharge coming from kittens eyes?

There is a feral cat that has taken up residence in our shed and she had kittens. They are now weaned and i caught them so when they are old enough they wont make more:). They are mitten toed and just adorable! But I noticed that all of them have this yellowish discharge coming out of there eyes and was wondering what it is and if it is contagious. If anybody has any idea of what it is and maybe how to cure it i would be very thankful! thanks in advance guys!!