If the restaraunt you chose has a banquet room, that is likely the place they will put your party for this. That can be why you can decorate. Many businesses have rooms for this sort of occasion. You can as the hostess, buy certain items from them for the party. Drinks, vege trays, snack trays, etc. so your guests only have to buy if they want someting more.
I had a jewelry party at such a place. I bought non-alcoholic drink, vege tray, and something else I forget, and the guests could order off the menu if they wanted more. A waitress was assigned to the room to keep track of what we needed.
Ask your Chinese buffet place can do something like this for you, and tell Mom what they can do, and that you have it worked out to be a surprise for him.