I can tell you now a Pharmacist is going to take the safe route and tell you he can't tell you anything if your child is under 6 yrs. old. If he told you that it was okay for you to give your child "such and such" medicine and it killed your child he would be liable. They shouldn't take the risk.
What you need to do is call the pediatrician. They can call something in but chances are they are going to want to see them to rule out flu.
Now, if you are talking about achild over 6 yrs.old then I buy plain liquid Sudafed, the fake Sudafed does not work for our family. We get it at a family style pharmacy but every one of them in my town carry it. Wal-Greens or Wal-Mart usually don't carry it. I give the kids 1/2 dose to start with. The Sudafed causes the vessels in the sinus area to do stuff and makes the congestion easier to get out plus it thins the mucus. I looked up decongestants up on wikipedia and it's interesting.
I also use Tussin, Wal-Mart brand of generic Robotussin for chest congestion, an expectorant. ALL over the counter chest congestion meds all use the same drug, it's called Guafenisin, and it's in robotussin, mussinex, etc...ALL the same med.
I don't use OTC cough meds because they just really don't work. The stuff from the pediatrician works. It lasts for a long time and it isn't that expensive in the long run.
I hope you little one gets feeling better very soon. Ours has puss coming out of his left ear so I know lots of stuff is going on due to weather and virus' and other stuff too. The pediatric nurse practitioner he sees says she has been working until 6:30 or 7 each night just to get all the kids seen for the past week or so.