When I was a kid, the only thing worse than being sick was the repeated blowing of the nose/smearing snot all over my face. Learning to blow my nose stands out in my memory as one of the most loathed phases of my youth. My poor nose would get more and more raw as the day wore on, and if I didn't already have a headache, you'd better believe I would develop one by the end of a day full of attempts to clear out my stuffed-up sinuses. As if that wasn't terrible enough, it turns out there is a legitimate reason to avoid blowing your nose, and it's very likely that you are teaching your child to blow his nose in such a way that could actually cause him harm.
Snot is just nasty, there's nothing pleasant about it. But your body's natural reactions to excess mucus—coughing and sneezing—cause very little pressure in your sinus cavities. Nose blowing on the other hand creates a ton of pressure, and actually shoots mucus into your sinuses with every blow. You can even send the infection spraying back into the middle ear if you blow your nose too hard.
Mucus serves the very important functions of keeping your nasal passageway and throat moist. In addition, it nabs foreign particles and germs we inhale before they have a chance to cause problems. Think of it as a sort of fly paper that catches germs trying to buzz in. As gross as it is, mucus is a big part of our body's defense against illness.
Ideally, we should keep our nose blowing to a minimum. But what about those days when your child becomes a relentless fount of snot? Should you just let it run? If you're past the nose suction age, you can teach them to blow their nose safely: one nostril at a time. When you hold the tissue to their nose, apply pressure to one nostril so air only comes out of the other side of the nose. On the next blow, switch sides. It's a simple thing that has the potential to make a big difference in the duration and severity of the cold.