We understand that you are trying to demonstrate the diffusion of the two gases, which react when they come together to form solid ammonium chloride. The concentrated solutions of HCl and NH4OH are both corrosive and should be handled in an operating fume cupboard.
Many models of fume cupboards have a purge system that keeps the fan running after it has been switched off to allow any residual hazardous vapours/fumes in the cupboard to be removed. Therefore, it may not be possible to turn off the fume cupboard for 10 seconds. In addition, we are not sure for your procedure whether 10 seconds would be sufficient time to see the reaction, even if you were able to turn off the fan in the fume cupboard.
Alternate method: We recommend an alternative way of demonstrating this reaction see: http://www.rsc.org/learn-chemistry/resource/res00000682/diffusion-of-gases-ammonia-and-hydrogen-chloride?cmpid=CMP00005016
The concentrated corrosive solutions will still need to be handled wearing appropriate PPE (lab coat, closed-in shoes, safety glasses and nitrile gloves) and in an operating fume cupboard, but due to the reaction being contained in a closed vessel, it can be taken out of the fume cupboard to show students. Another benefit is that smaller quantities of the concentrated solutions can be used. Cleaning up should be conducted in the fume cupboard: the cotton buds/balls can be placed in a beaker of water to dilute the reagents before disposal and the tube can also be rinsed using water.
'Diffusion of gases - ammonia and hydrogen chloride', Royal Society of Chemistry website. http://www.rsc.org/learn-chemistry/resource/res00000682/diffusion-of-gases-ammonia-and-hydrogen-chloride?cmpid=CMP00005016. October 2015.