Well, there's a limit to what you can do once they get sick. I would use this time to stress the importance of handwashing - sing the ABC song twice while lathering and rubbing, then rinse. A lot of kids put a dab of soap on their hands and immediately run them under the water - useless. And anti-bacterial soaps don't work on a cold virus, plus they create "super-bugs" that are antibiotic resistant.
For your son, try elevating his mattress so that his head is higher than his chest - sometimes than can help the nighttime coughing.
Mucus in the stomach can cause nausea but there's sometimes not a lot you can do if they have post-nasal drip - that doesn't get blown out into a tissue.
You could try some saline drops in the nose to keep things a little more open or to make it more tempting to blow out. You can also get a Neti pot at most pharmacies or places like Whole Foods - it's an ancient remedy that's coming back and has been endorsed by celebrity docs like Dr. Oz. It's harmless and might help. Depends on the age of the kids whether they can do it. You put in comfortably warm water and some salt. Then have the kids tilt their head over the sink or over a big pot or bowl, turned to one side. Gently pour the water from the pot into the upper nostril so it flows back and into the sinuses, then it drains out the lower nostril. With the water comes some of the mucus, and it helps cleanse the membranes. Feels weird and very "full" - when done, they'll have to blow out all the excess water. Do one side fully, then do the other. You might try it on yourself first to get the feeling and positioning right.
Otherwise, fluids fluids fluids - water, weak tea, etc. Ginger tea is helpful for stomach upset if they've got any of that with the mucus going into the stomach.
Then, their immune systems need to be strengthened. No one in our family has been sick for 3 years - not a cold, not an allergy, not a stomach flu, nothing. And my chronic bronchitis is a thing of the past. So it can be done if you learn the science and work with others who have had success! I just hang out with healthy people now and it's SO awesome! We're all exposed to the same things as everyone else, but these germs just don't take root and give us the hell you are going through!