I like 5 guys, the burgers are really nice, which ultimately is the main thing, but it is expensive, and for the price you pay yes you get nice food, but the ambiance is just like any other cheap fast food place. Perhaps that's not as much of a problem for 14year olds.

Turtle bay has a good lively atmosphere, that might be worth checking out, or if they're adventurous 14 year olds somewhere like wahaca or chai street