Poor little things.
A couple of years ago I had the cutest little Meezer mix (truth is, I still do *g*). This little guy had the worst URI I have ever seen in a kitten who survived. The vet gave him Amoxicillin, which did no good. I went ahead and put him on Clavamox. Give just one drop if he's little bitty (a couple of weeks old). I give a six week old about a 1/10th of a cc. One vet said, "Just wave the dropper in front of him."
If he's really stopped up or mouth-breathing I also buy drops of straight saline nasal drops (no decongestant additives of any kind). Now you get to find out just how strong a 14 oz. kitten really is. Buy a dropper with a little bitty tip and if you can do it, put a drop in each nostril. Stand back because the snot is going to fly. (I hope that's not too crude for yall.) This is one of the most difficult things a foster can do, cuz they're fighting for their lives. They don't want that stuff up their nose.
I also set up a vaporizer and put Vick's vaporizer liquid in the moat. I don't know why vets don't recommend this more often. The moisture really helps loosen things up.
Keep his little nose wiped and clean. That mucus is caustic and you don't want his skin getting irritated on top of everything else.
If they're weaned and not interested in eating, we have a few forced meals of a/d. I always keep some on hand in case of an emergency.
Keep em warm.
Anyone have anything else to add?