documents needed for a guardian of my niece

my brother works in africa (ofw) he and her wife will have a vacation here in the philippines,. their connecting flights will be in hongkong from africa then to Philippines. they wanted to have 3 day tour in HK and they wanted their 2 sons here to be with them. and i will be the guardian of their sons to go to HK. what documents needed for us to possibly go for a 3day - 2nyt?