Casual lunch between Philly and Kennett Square?
What day of the week? There isn't a whole lot along that route, and once you're in Chadds Ford you may as well just keep going into Kennett Square. If you're going on Friday or Saturday the Booth's Corner farmer's market is interesting and has several good lunch spots.
I really like El Gran Rodeo at the intersection of 202 and Route 1. It is located in the Painter's Crossing shopping center. I enjoy both places and have not had a bad meal at either one. I actually think you have more of a selection at El Gran Rodeo than you would at Taqueria Moroleon.
May I suggest an alternative. I can't really help if you want Mexican Food, but if this is a "friends' lunch" you might prefer it
Not much shorter in distance, but Longwood Gardens has a pretty good cafeteria style place (it's better IMHO than the sit-down restaurant)
Then you could stroll the gardens while catching up.
In Chadds Ford, you have Hanks (simple, fun, locals), The Chadds Ford Tavern, a new place, Bistro (next to and conneceted to Brandywine Prime). And justr before 202 on route 1 you have a whole array of places in Brinton's Bridge shopping plaza - PF Changs, American Grille, etc.