Amongst bi-lingual people I know I don't know another family whose DC use their second language as actively as mine. Many people say they always speak German to their DC but they answer back in English. I only know one family who uses 3 languages quite actively, but their DC speak English to each other.
My DC speak German to each other, always, even if they are out with DP who does not speak German. They will speak English to him but never to each other, and I always speak German to them. DS (Y3) said to a friend once that he always thinks in German, even in school.
I do OPOL with DC quite strictly and there was only one or two situations when DS started school when he spoke to me in English - I just let him repeat it twice and then said it back to him in German. It hasen't happened again, he always tries his best to speak German and sometimes if I 'correct' him (repeat a senctence back as a question with the correct vocab in it) he will immediately use it.
I always translated books into German when DS was little, but DS was never much into books. With DD (4), I read to her in English as well (but she knows I read and that its not language coming from me, but from the book), when we discuss what happened in the book it will be in German. I do that because DD loves books and I couldn't possibly keep up with her thirst for reading if I tried to translate it all. She is not at school yet and her first language is definately German.
We use German websites and watch programmes over the internet, and when there is a choice of languages, e.g with a DVD they both will choose German.