Where r u based in hong kong ?
There are many venues but expensive/ inexpensive is subjective. In hong kong side u have my gym , Playtown, Wisekids , gymboree which rent their places including activities for 2 hours. I don't remembertheir minimum charge and no of kids but u can call them and find out.
On Robinson road there us a hotel bishop lei and they have a hall that they rent out they have toys and a bouncy castle and is more reasonable than others.
Best of luck