Have you checked whether the place is available on the day you want to book it for ? Must check imm - quite short notice already.
My son had his B-day party in McD, so did a few of his classmates, so we've been to quite a few McDs all with different styles. The kids never grow tired of it. We've been to Pizza Hut and KFC parties, and in staff clubhouses of Hang Seng Bank and New Asia College CUHK.
If your 1st choice venue is not available, apart from thinking of McDs, you can also consider holding it in function rooms of hotels, e.g. TST YMCA, Eaton.
If the kid wants to go to a B-day party, they will go wherever you hold it. So your own clubhouse might also be OK. Just give everybody an invitation card + map + RSVP deadline.
You may consider 冒險樂園, HK$40 per children including drink, little snack, 20 mins 波波池, 1 hour function room and .... but need to pay HK$100 for cleaning fee, call 25600003 for details information.
I highly recommend the Children's Technology Workshop. For further information, see the "Birthday Party" section in this website or call the Workshop direct.