Check out Spontaneous Celebrations - they have a great place for large groups.
You may want to check with the City's BCYF community centers in your area to see what they have available.
The Roslindale center has that capacity and the rental rate is reasonable. You must set up, clean-up, and break down on your own however. Space is usually allocated on a first-come basis. No alcohol.
They are at 6 Cummins Highway (corner of Washington St) in Roslindale Sq. 617-635-5185. You want to speak to administrator Cynthia Johnson.