Hi Sticky,
Just seen the thread now. I've been living in Berlin for 18 years and both dcs were born here. We've spent 9 years in total with childminders/Kindergarten and seven years so far with the school system.
There are plenty of options for English language education here. You do need to decide if you want to go for a state school (which is truly bilingual, although native English speakers with no German would be accepted at 7) or a private English-only school.
If you are in the state school system and living in Brandenburg (the federal state surrounding Berlin) it can be tricky getting into a Berlin school - officially you're not allowed to do it and some people rent a cheap flat in Berlin to give them a Berlin address.
Some good bilingual state schools are:
Nelson Mandela State International (or something like that)
Charles Dickens Europe School
Quentin Blake Europe School
All three of these are 'Ganztagschulen' from 8 am - 4pm.
There's also the JFK, but as non-Americans you have virtually no chance of getting a place.
Other private schools are officially bilingual, but in practice they're full of Germans who want their children to learn English early on. Examples are Phorms and Berlin Metropolitan School. I've heard that the Kant Schule and Berlin International are both quite good, also private.
International School Villa Amalienhof is English language only, and as pretentious and overpriced as it sounds. And you said you were more interested in a bilingual school anyway.
There are also a huge number of bilingual nurseries, again most of them are German-only kids whose parents want them to learn English, so dd2 might feel a bit left out (but would learn German very quickly!). My younger dd went to Tom-Sawyer-Kinderhaus, which is Montessori, and she loved it. It's run by a non-profit organisation but the fees are the same as for a state nursery, and very income dependent. We never paid more than 180 euros per month, and the maximum is 500, I think.
It's much easier to join schools/Kitas in August/September at the start of the school year, of course, but internationals schools are more relaxed in that respect. The registration date for Kitas each year is in February, I believe.
Two good areas to live would be Zehlendorf/Dahlem and Westend. Both have bilingual schools nearby and many other English speakers, especially Zehlendorf, which used to be the American zone. The local officials at the Jugendamt (for Kita and school registration) can also speak better English there, through more experience.
If you want to send me a message (I think I've activated that), I can give you more details.