In my family (including extended family), everybody speaks both German and Danish. It also means that everybody can speak whatever language they happen to prefer, and we even throw in a word from the other language if it carries the meaning better. We live in Austria so German gets lots of exposure.
I asked this questionthis question about speaking to my children and some of the answers mention the idea of requiring the children to respond in the same language. I'm not sure whether that's a great idea or not so I thought I'd collect some thoughts on this.
On one hand, it will probably help the child to a better active vocabulary, but on the other hand I don't think I can actually make my kids do this. The kids know that I'm fluent in both languages, so why should they be forced to speak Danish to me? Especially given that they see me using whatever language I happen to prefer at the moment (except that I always speak Danish with the kids).