How old is the youngster concerned? IIRC the option for turning off the passenger airbag is for occassions when a rear facing child seat would need to be placed in the front seat. Obviously having an airbag catapult a child face first into the back of the front seat would be counter productive to safety!!!! If the child is to be seated in a booster type seat then I see no reason why the functioning airbag would be cause for concern
If, however, the need arises then I am sure the airbag could be unplugged from behind the unit itself. This would mean disconnecting the battery, removing the glovebox to enable removal of the airbag to unplug. It would all then need to be put back together again and you would probably, as previously mentioned, end up with warning lights on your dash!