My son has the same problem - very sensitive nose.
We also gotten an air purifier as well as a humidifier. We got them seperately. The brand we got was Maier. You can google for their website.
We intended to get the Panasonic one at first but it was outside our budget.
Ionizer basically is a negative ion generator. Air borne particles attach themselves to these negative ions and then these ions are -ionized by earthing themselves to walls and such. So basically, they're pretty harmless. Ionizers are NOT ozone generators. For more info, visit this site: Air ioniser - Wikipedia, the free encyclopedia
I think the salesman at Best Denki confused negative ions to free-radically charged ozone molecules.
Anyway, the air purifier did help with our son's nose. As for the humidifier, it didn't help as much.
Most importantly, for sensitive noses, there're triggers. For my son, it is dust. So we also bought a dustmite vacuum to use on his mattress every month. My husband also washes the air con filters every week.
I think it is only by finding the root cause of the nose blockage, then you will be able to effectively stop your child's nose from being blocked.