Hi from Argentina!
A few yeas ago when my daughter was young, we teach her English using DVDs with subtitles in English. So she could choose to see the movie with the voices in Spanish/English/Portuguese and the subtitles in Spanish/English/Portuguese. The more sane configuration to learn English are the sound in English with Spanish subtitles or the sound in English with English subtitles, but sometimes she used other configurations for fun.
Can you get a few DVDs with English/German combination? They are probably difficult to find now, and the DVD zones are painful. But perhaps you can use the idea with another implementation.
PS: From the experience of a few friends that live abroad, the real native language is the language in the kindergarten, not the language of the parents.
