I believe they typically use the Japan Terrace for smaller Dessert Parties. My fiancee and I have a Dessert Party booked for September 29, after our wedding and we'll only have 6-8 people attending.
The only bad thing is, you still have to pay for the minimum 20 people, even though you have much less than 20 people. The cheapest Dessert Party menu option is (I believe) $18.50 per person x 20 people = $370, plus other applicable service costs.
Good luck!