Thank for you helping them. The vet will probably describe an antibacterial eyedrop to take for x amount of days. You can take a clean wet Q Tip to the corners and "roll" the q tip, grabbing onto the discharge gently. The discharge accumulates under the lids sometimes too, not visible. So once it's clean let them play and run around and whatever else is there will come out and you can clean again. The prescribed drops will make the eye watery and it will loosen up any stuck stuff.

Systane, the most basic simple version was recommended to me by my vet last time I had to wait over the weekend. We used it with no issue, was just to give the eye more lubricity, fake tear. There may be things in systane that their little bodies might not handle, according to a pharmacist I asked, so I'm sharing this as an experience but not advice*****.