Along with Sqad's cogent advice... if any of the kids are in school (a 5 year old here in the U.S. could be in kindergarten) then chances are they are bringing the virus home and spreading it to the other children.
As already stated, a humidifier works wonders in winter conditions due to the higher temps and dry air by introducing more... well... humidity... which is a good thing for the kids breathing apparatus.
In fact, we've found that anyone (might be difficult to start young children) regularly irrigating the sinuses to be one of the best ways to cut down on catching the particular strain of cold going around (plus religiously washing one's hands). We use a product here called "Oceans", but it's just a squeeze spray bottle of saline solution... no medication of any kind. Anyone (adults included) should use the product as often as possible, but at least 6 to 8 times daily. Just spray one "shot" in each nostril and sniff inwardly 2 or 3 times immediately afterward. Really work it up into the massive sinus cavities all humans have.
You'll notice that most colds announce themselves with a sore throat... which is the mucuous tissue in the throat responding to the sinus drainage. In fact, if you'll look at the childs throat with a flashlight (excuse me, a "torch") you'll see red streaks running down it, which is the "trail" left by the virus and bacteria from the infected sinuses. Having said that, try to avoid using prescription antibiotic medication... for yourself as well as and especially children. The overuse of such medication, in my humble, non-medical trained opinion, primes the child for future infections. (Disclaimer: A doctor's opinion prevails, however)
Good luck... take an aspirin and plenty of water... both for you and the kids... Best of luck!