Althought there is no waterpark in Hong Kong, you could enjoy the water slide in Public Swimming Pools with a reasonable admission fee.
Many public swimming pool are designed with leisure pool for kids, some of them even get slides like these.
Herewith the Opening Schedule
Summer (April to October)
1st Session: 6:30 am - 12:00 nn
2nd Session: 1:00 pm - 6:30 pm
3rd Session: 7:30 pm - 10:00 pm
(Session breaks: 12:00 nn - 1:00 pm ; 6:30 pm - 7:30 pm)
Temporary Closure for Annual Maintenance
From 1 November to 31 March of the following year
The cache
It is located outside the paid area, you do not need to go inside the swimming pool.