I made an interview to two friends whose daughter is bilingual in Spanish and English. The husband is from England and the mother is from Spain and both are fluently in English and Spanish,the daughter, Natalia, has 7 years old and she has been educated in English and Spanish languages. Now they spoke English in the house and she learns and uses Spanish in the school.
When she born they planned speak to her in both languages, one parent one language, her first words were a mixture between English and Spanish and everything seemed to work well the first years.
When she was 2 years old she became to spend a lot of time with a English speaker babysitter and her English level was higher than the Spanish. They were a little concerned about this because they were planing her starting at the Spanish school and she didn't seemed to respond to Spanish language. But when she started the school she began to speak more in Spanish, probably because it was her playing-language , and her peers spoke in Spanish only, so she began to speak in Spanish in the house and she always started the conversation in Spanish.
With 6 and 7 years she has began to understand the language rule in her life and she doesn't show any problem in switching the language with the mother and the father and with the community and the family. In the house the mother speak more in English to support the minority language. Fortunately the daughter has many opportunities to speak in English outside the house.
The process of teach bilingual kids can be hard and sometimes frustrating, but parents need a lot of knowledge about the bilingualism, patience and some times professional help. It depends too on the personality of the children bacause in some families the responses of the children were very different when using the same estrategy.