I have a 3 year-old dd, born in France to American mother (that would be me
) and a French father. She lived in France for 2 1/2 years with no other exposure to English except with me. Now we live in Central Asia where the official languages are Russian and Kyrgyz, with a heavy leaning towards Russian in most settings.
I have always spoken both English and French to my dd. I feel comfortable enough with my French that it doesn't feel "not intimate." Dh and I speak both French and English together - very often he'll speak French and I'll answer in English. My dd has been hearing this since birth.
Because of the non-English atmosphere in France, she definitely preferred French, even refusing English when spoken to her, as in "No, maman, say it this way!"
Once we moved here, she became much more exposed to English. She started to speak it as though she had always been speaking it. She still prefers French, but will hold conversations with me in English. She also picks up on conversations between other English-speaking adults and me, and will make comments in English on what we are saying.
Now she understands Russian very well. It's amazing! Sometimes she will answer a question in French, but she answers it correctly! She probably has about 20 words in Russian. She was in a Russian preschool for about 2 months, and since then she has had a French-speaking babysitter who inserts some Russian here and there. But mostly it seems to have come from interactions between Russians and my dh on the street.
Personally, I wouldn't use German as a conversational language. I would choose an hour each day where you sit down and point at objects and name them in German. There's no reason you couldn't impart your new knowledge on her; just give it some very specific structure. Then when you're out and about and you see a dog, you can say "Do you know what that's called in German - or I actually say that part in the foreign language, so "Do you know what that's called auf Deutsch?" "Hund!" etc. She'll pick up those words in no time, and like the above poster said, she'll at least realize that there's another language floating around besides her own.
If she's open to change and challengning situations, she might enjoy a German preschool a couple of hours a day. But it's difficult for most children to be thrust into that kind of situation.
In terms of how well my dd speaks her two main languages: I think she'll be slightly behind if and when we go back to France, mostly because she's missed out on the cultural use of the French language, if that makes sense. But I have no doubt that within the first month back home, she'll be caught up. Same goes for English.
The one problem some bilingual children do have is that they have difficulty writing well. They often have to stop and sort out their thoughts. I think my dh is very wise when he says that could be a good thing.