Kid Friendly with Great Food!
There are lots of options..any particular part of the city you will be in? price point? More info would allow us to give better recs.
I second the Reading Terminal Market recommendation. In Chinatown, Rangoon is a great option. The food is Burmese, something not so easy to get everywhere. For Cantonese style Chinese, Sang Kee is great and easy with kids.
In West Philly, Distrito is a great option for more upscale Mexican. They have booths and lots of visual interest to keep a little one busy, and would be fine for lunch or an early dinner.
Do go to Reading Terminal Market for breatfast at the Dutch Eating Place for delicious pancakes and apple dumplings. It's best to sit at the counter if you can swing it with your child...it's a lot of fun. Also, the Market is a great place to go for lunch, as they have a number of and see all of the food stalls, since they also have really good baked goods, soft pretzels, and chocolate confections there.
Parc on Rittenhouse Square is always a good place to go with kids. Jones is also kid friendly but the food is a step below Parc IMO. There are many places in Chinatown that are very child-friendly (I am sure someone will post the better options) and you could always get a good pizza at a place like Zavino.