Where do I eat in Philly? Kid friendly, gluten-free friendly, etc.?
I live outside Philadelphia and wish I could help; I do not eat gluten-free and haven't worked downtown in years, etc. I found this site:
http://www.glutenfreephilly.com/p/gluten-free-restaurants.html which might be helpful - but keep in mind some of these places are 2 hours from center city (stick to ones labeled Philadelphia) and some are pretty upscale and not really kid-friendly.
How old are your kids? Are you staying in center city or in the burbs?
The Reading Terminal Market is a Philadelphia institution that is worth visiting and there must be some GF eating there. Seating is very casual and hard to find at lunchtime; I'd recommend going on a weekday at "off" hours, i.e. not 12-2. Chinatown is right there and you might find places that recognize GF issues.
Capo Giro gelato is unbelievable, and they are generous with sampling before you order. I highly recommend.
I hope more Philadelphians reply with specifics.... good luck!
Gluten free, I don't know. A lot of people are going gluten free even if they don't need to. But how about http://www.jacksfirehouse.com/ Jack McDavid partnered with Bobby Flay in the early years of Food Network and he's a damn fine chef.
Definitely go to Reading Terminal! It's wonderful. The Italian Market in South Philly is also really fun to wander around.
The Philadelphia restaurant scene is great and a lot of it is BYO so it can be more affordable than some other places. Definitely not gluten free, but my favorite pizza place is Mama Palmas at 23rd and Spruce - the pizza is amazing and it's definitely kid friendly. Also in that area, at 23rd and Pine, is Dmitris - Greek seafood tapas/family style. They actually have three locations and they're all really fun. The one in South Philly is BYO and very small, get there early, but the 23rd street location isn't BYO and I've never had a hard time getting a table. I bet they're kid friendly if you go early - just depends on how adventurous the kids are! Get the octopus.
I've found the Steven Starr restaurants to be generally good. Buddakan, which is Asian-Fusion, is great. Pod is fun too, and also good food, especially if you like sushi. The only poor food experience I've had in his restaurants was at Frankford Hall - fun atmosphere, but dinner was no good. I get Indian food at New Delhi in West Philly, which is great (and also wonderful delivery). Also around 40th street is Distrito, which is kind of a funky Mexican tapas place and also really good. Amazing ceviche!
My absolute favorite dinner place in Philadelphia is Bistro la Minette, which I think is at 6th and South. It's kind of French-American, small, family owned, and delicious. Bistro 7 in that area and same genre is also really good. In West Philly I like Marigold Kitchen for bistro-type food, but I think it's a bit pricey. Recently I've been underwhelmed by Bliss (on broad street) and Zahav (in old city), but that's just my experience.
If you're interested in brunch (I know a lot of people aren't but I love brunch...) I recommend Parc on Rittenhouse for high end, and Sabrinas (they also have a couple locations, I think one near Penn and one in South Philly) for your classic giant plate meals. Both delicious. My personal favorite neighborhood place is Day by Day, which is on 21st and Locust (can you tell where I live yet? :) It's also BYO and they'll bring lots of OJ if you want mimosas.
I hope this helps! I grew up outside Philly and I live there now so if you have any other questions I'm happy to give you my opinion of anywhere I've been if you're interested. Have a great trip!