Ruck Over
When the revolution comes, pants will do you no gd
That kinda sounds like the group I had for my rehearsal in the same neighborhood. I went to Nodding Heads (which is now defunct), but I would lead you in the direction of a restaurant/bar.Parents want to pay for it and they are both retired, we arent the type that would opt for an upscale place...would be about 20 people. No preference on Food.
Speak to a manager, and they should be able to carve space out for you pretty easily or give you a private room, pending on the day and time you are looking. McGillan's Old Ale House has an upstairs that would fit well. Smoking Betty's is a nice spot, have had work-office lunch events there, a retirement party (up stairs). On the more casual-scale, the Field House has both an adjacent side room, and an upstairs area for a party of that size. Given the address of 12th & Market, you're likely at either the Marriott or the Loews, both of which could accommodate. A bit further walk down 12th (Pine) is Mixto, a very fun restaurant with an upstairs that could be group reserved.
For a good meal, some space between your party and everyone else, and very local to 12th & Market, these are decent rec's. Would probably list in my personal desire as McGillan's, Mixto, Smoking Betty's, Field House (location/space, not cuisine). If you are a bit more adventurous, there are plenty of options in Chinatown as well. However, I'm unfamiliar with those venues to recommend.