I have spent an afternoon at the playroom in Tsing Yi that Carang mentioned above. It is great for kids. Enclosed and two rooms full of toys, books and games. It is also nice and clean and gives you the freedom to organise the party at you want but at a children friendly environment.
You could also try Kids Gallery. I went to a fourth birthday and the do lots of jumping and dancing and then each child 'made' a fishing game with lots of paint and glitter -- very cool. they have lots of locations around HK I think. They have a website too.
we use to attend classes there before my daughter turn 1 cause she was too young for the children facilities in public parks. And we also attended 1yr old b'day party. Great fun.
Not sure, but they blocked off the entire seating area from Wild Fire up to the Gelato place. Asked one of the wait staff and she said this was only the 2nd birthday that had been organized there. So I guess not a lot of people know about it.