I had also found out the oregano oil (Oreganum Volgare) has the ability to cure sinus infections.
However, throwing caution in the wind....I would not recommend using oregano oil for a prolonged period, it may have a side effect, which would to cause issues with the liver function.
Whenever the sinuses flare up, using it till they are ok, sounds like a good practice.
For a child, I'd personally suggest diluting it with olive oil, since the oregano oil is already being diluted with parts of olive oil.
As for inhalation...perhaps that is something not to be done for a long time, either. If so, putting it in a cloth or something to inhale the vapors, which I found was suggested by an oo home page, sounds worth trying...just don't have skin contact, the oo may cause irritation or burning to the skin.