Wow, thanks for all the answers, there's certainly much to think about when there are children involved. We too have adopted the system that in the house it's English and outside it's French (obviously with exceptions when there are friends around). We're lucky here in Montpellier that there is an association that teaches English to English children in a saturday morning club (www.ieam.info) and I've heard of another in Civray (www.accents-asso.fr) but elsewhere it seems pretty hit or miss.
Am I wrong but do only the children with both parents being English/Anglophone have the right accent in both languages? Mine children sound like either English kids or French kids depending on the languages whereas children with French-Anglo parents seem to hold onto the French accent even when they are speaking English. What do you think?