Hi Vera,
I have heard that some English-language playgroups exist, but haven't been able to nail anything down myself. I think there is an AWO one at least one day a week.
Our kids are being raised bilingually as well (English-German). Our son is a bit over 3, our daughter is not quite 2.
If you poke around the threads on this site you'll no doubt notice there are various English-language groups in Hannover as well.
This thread in particular (click here) talks about them. I organize two of them -- H4ES and the Anglican church services -- and also sometimes do book readings for kids at Escape Books (like I am tomorrow), so maybe we'll run into each other at one or the other place (muahahahaha, there's no escaping me, run while you can *cough*cough*).
Anyway, would be a pleasure to meet you when you get here.

Cheers,
Fairfax71