My oldest cat had a cold before, kept on sneezing. Since I did not have the money to take her to the vet, I just kept her warm on her pillow away from drafts. She did improve and thankfully it was minor. You may try feeding them warmed up canned food? Or put some nice warm broth in a dish for them to try? It's something, a start.

When mine had a cold, I took him into the bathroom, turned on the heater and the hot water to get the room steamy. I did this for about a week and it seemed to help.

I had also taken mine to the vet and gotten antibiotics.