Good Mexican near Balboa Park or the Zoo? Anything in Oceanside?
Mama Testa's is very family friendly, very good and close by. Tacos only.
Limited veg menu.
A bit further (about 5 minutes) is Rancho's Mexican food. While the food is good/mediocre it has a HUGE vegetarian/vegan menu. Anything on the menu can be made vegetarian.
A block further is El Comal that has the very limited veg menu but the honor of having the best vegetarian burrito in town (with rajas, mushrooms, crema, beans, and rice)
About 2 minutes further is Mariscos German Taco Truck (tacos only, seafood only)
About 3 minutes past that is Super Cocina (the best Mexican food in town) but you might have a hard time finding something vegetarian there.
Just to clarify, the Mariscos German truck is not tacos only, although that is their most talked-about item. They also do (taken straight off of their menu) clamatos, cockteles, botanas, tostadas ceviche, and burros, not to forget their wonderful (and complimentary) seafood consomme...