We have five kids, Jonathan 12 years old, Erik 11 years old, Nicolas 8 years old, Stephanie 5 years old, and Oliver 4 years old. When Jonathan was little we decided that the best solution for us would be that we speak English to him and let him learn Swedish from other people. It worked splendidly. At that time "experts" said that each parent should speak their own language. But we felt that since Stefan speaks English so well it would be easier for us to be consistent at home. Jonathan learned both English and Swedish and by the time he was three years old he could translate for Farfar (grandpa)! The other children have also learned both languages just as naturally. Some of them have been a little older before they started talking but then both languages came at once. They have an almost instinctive sense about who to speak English to and who to speak Swedish to, that seems to amaze people. And they have no problem switching from one language to the other within the same conversation.
Now that the three oldest children are in school and spend most of their days speaking Swedish, we see that Swedish is the stronger language even if they have learned English first. We work on not mixing the languages in the same sentence. Sometimes they (and I) will say the first word that comes to mind and it isn't always the right language!! To be honest, it is not always that we even hear that they are mixing languages, but other people notice.
Jonathan is now in the fifth grade and his classmates are beginning to study English. He joins the sixth graders for their English studies, that way he will learn grammar and spelling.
We have never had a problem with them answering in Swedish instead of English. But sometimes they do forget and will say something to us in Swedish, and we just remind them that we speak English at home. We have also explained that it will be very valuable for them to be fluent in English, even if right now it is something they take for granted.
When Oliver was born I was given some pamphlets for bilingual parents. Now the "experts" recommend that families speak the weaker (that is to say the foreign) language at home, even if one of the parents is not completely fluent. So I guess we had the right concept all along. It has at least worked well for us.