This is Kel's Books Alto Palermo location. It's LOADED with kids' books in English.
It's not exactly on the Benihanna side of the "west" half of the mall (but rather the Santa Fe side ...vs the Arenales side), but it is absolutely IN THE BASEMENT!
"Subsuelo", "Piso -1"...whatever you want to call it.
It's kind of broken into 2 or 3 sections with at least one of the sections being out in the walking area kind of like a kiosk (NOT "kiosko").