No, no, no, this is an awful idea!
I love Reading Terminal I would strongly suggest not taking a group of 20 people there, much less a group of 20 kids there. It’s an well over 100 year old farmer’s market under what was the Reading Railroad terminal headhouse in Philadelphia. There are some great places to buy raw foods to take home & cook; including some things I only get there as I don’t know anywhere else that sells certain things, some Amish food merchants, some great dessert places, & a number of places to get something to eat. The aisles are 2-3 people wide, & with people standing in line to order from various merchants it can be tough for two or three people to stay together walking thru the market, let alone one or two wanting to order here & some wanting to order there, etc. The middle section has open seating, but it’s mostly 4-tops & it’s first come, first served. There’s no way you’re going to get everyone to sit together.
On top of that, Spataro’s has been in the market for decades; they were known for their deli sandwiches; cheesesteaks are a relatively new addition for them when they moved to their current location. I’ve been there & seen 20-30 people in line waiting to order or pickup one. I’ve never had a cheesesteak that’s worth that wait.
Second Campo’s or Sonny’s as they’re easier to get to but even there you’ll kind of overwhelm them.