below are those party entertainers I met before:
1. http://www.madscience.com.hk/bday.htm
2.http://www.funhousehk.com/
3. http://topaz.hknet.com/~zani/children.html
For "1", it's fun and more suitable for elder kids, say age 6 or above. The company will send 3 persons (the party size is around 20) who will guide the gangs to do some experiments, making ice-cream and cotton candy, etc.
For "2", Rachel is the person-in-charge. I saw her show in my kid's school x'mas party. She acted as a disc jockey and introduced games mainly for dancing and singing. The kids loved it so much but if you are looking for other activities, I don't have much comment here.
For "3", Zani is the clown who plays magic, tricks and games same as those other clown's. His price is more reasonable comparing with others.
You can search below web site for more choices.
http://www.geobaby.com/directory/hong-kong/party-supplies%2c-entertainment-and-venues/