Schulte
You have to go hardcore!
If you really want your kids to be bilingual you have to facilitate it. Despite what friends / onlookers say ("oh, how lucky, they get to learn a second language with no problems at all!" is one I hear a lot), if you are the prime or only source of German, it is your responsibility. You may or may not want to shoukder that responsibility and as a parent, I am sure there are lots of other demands on your time / energies.
But, if you want it to work, and by that I mean really want it to work, then the strategy to adopt is HOPOL ( Hardcore One Parent One Language
).
As of today, you:
--only speak German to your kids no matter what, in front of their friends, your friends, adults who don't speak German, shopkeepers, doctors etc. At the moment, you are sacrificing your own children's chances at balanced bilingualism in favour of politeness to strangers.
-- if your children speak back in English, no problem, but do not switch to English
-- all DVDs, CDs in the car, storybooks are in German. They might not like it, but then many kids don't like brushing their teeth but good parents put dental health above their children's preference not to brush regularly. Same applies. They might whinge for a fww days, but remember you are the parent and you can see the longterm benefits (to teeth brushing, bilingualism etc.) that they cannot see and cannot be exoected to see
-- if you can afford holidays abroad, they need to be to German speaking places. Doesn't need to be Germany. Find a campsite in Southern France which is popular with Germans, find the equivalent to Club Med where German will be the main language. Plus, of course, visit family in Germany if possible.
-- What interests do your kids have? Do a deal with your kids with regards to their own interests.. Unlimited Pokemon cards, Lord of The Rings DVDSs or whatever it is, as long as it is in German. They don't have to be reading highbrow German literature or even high quality German children's fables to get a good grounding in the language. Let them choose the topic (even something horrid like wrestling!) and keep them supplied with books, games, DVDs etc. In German.
-- Don't overcorrect their German when they do speak to you. Let them get used to speaking and in time you can sort out grammar, pronunciation. Remember, monolinguals make mistakes ("she threwed the ball" etc) and parents gently reformulate without assuming their monlinguals will never learn to speak properly. Give them time for them to transition in to the new German routine.
-- Play word games to build up their vocab.
I personally don't think bilingualism comes effortlessly in most families. The successful bilingual families I know are the ones where the parents have worked at it. That's not to say it is onerous, but it takes a bit of thought and a lot of determination.
Good luck.