D.B.
Not sure what your budget is or what the activities will be. There are open fields up behind the Senior Center (Rogers Road), and there's a pavilion/picnic area with lots of space for frisbee throwing and so on at the Ellsworth-McAfee Park on Route 135 just over the line into Northborough. Call to find out what the deal is with reserving the picnic pavilion. Both Hopkinton and Ashland State Parks have picnic grounds (with grills) and swimming facilities/lifeguards. There's a per-car parking fee but not a per-person fee. You could call the town and ask if you can do something at the Lake Chauncy Beach if you are off to the side. Everyone has to show proof of residency for access, I think, so that's a lot to organize. They aren't going to take kindly to kids this age running all over the beach area where the little kids are though, so there will have to be adult supervision and some decent standards of behavior. You will need a serious clean-up crew at every location.
You could also pair up with another family. The kids are going to a zillion parties and there's also the graduation all-night program. You're unlikely to have 50 kids at one time, frankly.
If you want something indoors, you could consider the bowling alley in Shrewsbury but they'd all have to come at the same time for you to have something organized. In my experience, the Westborough parties are a lot of drop-in and move-on-to-the-next party sort of arrangements.
Restaurants that have large rooms are going to be expensive - are you looking for that sort of thing and the chance to serve a regular meal or buffet?