It could be from particulates in the air, like dust or inhalation. Where are they sleeping or spending their time?
It could also be from an infection in their sinus region. The sinus is inside the head behind the roof of their mouth, and when it gets an infection, can cause eye discharge.
Most likely of you can rule out particulate inhalation, it is an infection. If it is viral or bacterial, usually chicks are immune until reaching a certain age and size. How old are they?
You can try to open the first ones mouth and look at the choanal slit on the roof of their mouth. Look for abnornal coloration or buildups, but the problem is internal.
Make sure you keep clean waters everyday, and keep a clean and dust free place.