I did a masseuse course and few years ago and discovered this technique which will often cure a blocked nose when you have a cold.
I would say in works in about 90% of cases and takes about 5 - 10 mins. I have done it on myself, DH and DC from the age of 10 years. I'm not so sure how if it works on a streaming/running nose.
First, work out which side is blocked and work on that side.
Basically, all it is is massaging the area around the nose. I usually use my forefinger and make small circular motions on the skin. The idea is to push the skin, so your finger stays in contact (ie. doesn't slide over the skin) - hope that makes sense? KEEP IT REALLY SMALL AND GENTLE.
I work the side of the nose, from the nostril, and over the bone and up to the bridge of the nose. I will also do the area to the side of the nose, under the eyes i.e over the sinuses and above the eyes on the forehead. Another good move using two fingers is to push the skin from below the inner eyebrows up towards the hairline, BUT SMALL AND GENTLE MOVEMENTS.
All massage moves blood and lymph around and I think this is what makes is work.
I find having a blocked nose makes me really miserable and gives me a headache, so anything that helps I will give a go.
I cannot think of a reason why it can't be done on young children as long as its kept very gentle.
Please post if you have any questions, and if you find it works, then pass it on.