You should prepare one just in case. There is a better than even chance that no border agents would bother to ask, but if they do, it will cause your helper and friends some unwelcome delay, and possibly miss their flights.
Your letter should follow the typical HK authorization format:
"I,
, holder of , parent of , holder of , hereby authorize , holder of to take my child to travel in from the period of . For questions or clarifications, please contact me at "