The big theatres usually show kids' movies in spanish and english, I'm not sure about Martinez specifically but you could always take them to Hoyts in the Dot shopping center! http://www.hoyts.com.ar/cine/Hoyts-Dot
Actually check the language of the movie before you go. Not all movies are in English and subtitled. Children's movies are generally dubbed into Spanish as kids can't really read or if they can, read fast enough to enjoy a kid's movie.