Continentalmama I'd say mix as much as possible with native speakers of the local language.
We use English as a family language despite DH being German because we live very rurally and every element of the children's lives happens in German. Slowly as the older two children are now secondary age more German has crept in (especially school specific language - maths terms particularly) but even now German DH speaks English to the children 75% of the time even if I'm not home.
The children's German was actually stronger than monolingual peers when they started kindergarten and school, especially grammatical structures, according to their teachers.
What I made a point of doing from the moment we moved (when I spoke about 20 words of German) was hanging out in the local village playground for hours every day, joining multiple baby and toddler and music classes, inviting neighbours with children over, really making a job/ mission of integrating my children into the local monolingual German/ (and arguably though less successfully as "outsiders" meaning German speakers who can't trace their ancestors back 99 generations within the same village, the bilingual German/ Bavarian dialect) community.
I did join an English speaking toddlers group at one point but all my children's friends and 98% of their social contact outside the family has always been in German.
We did also have Oma and Opa, neither of whom spoke English, when they were small, though they lived an hour's drive away so that was contact once every 6 weeks or so.
My children didn't actually go to any form of childcare until they were 3, when each started kindergarten the month of their 3rd birthday. The research I did emphasised the paramount importance of a very strong foundation in a well spoken, gramatically solid native language coupled with daily exposure to the second language, and the also great importance of all languages the children are exposed to being spoken well by native speakers. I had to be very clear with kindergarten not to try to speak English to DD (she was the first non 109% German- Bavarian child to attend the kindergarten and a few teachers wanted to try their school English on her but I had to ask them not to).
So I'd absolutely agree that you should not speak a language badly to your children. Although my German is now fluent enough to work and study in a monolingual German environment my accent is really strong and my grammar often still wonky. I never speak German to my children and they speak perfect (though not age appropriate in that they lack teenaged slang) English and perfect (and age appropriate complete with slang) German. They don't use much dialect although they understand it though, dialect seems to be a separate issue and a big divide. Lots of local Germans who simply don't have multiple generations of ancestors from within a 10 km radius also don't use dialect though.
Good luck - you do have to work at avoiding an English ghetto and at keeping the passive English vocabulary building through reading aloud.