OP where I am there are international schools and smaller local private bilingual schools too which are also run by French speaking / local people. Honestly, all the English speaking children there, even after a few years, do not have a very good grasp of French. It's basic at best, which yes, may be sufficient for what you hope to achieve but I think you need to be prepared for this to be a huge upheaval in their little lives for relatively short-term and small gain.
I am the first to say that living abroad is a fantastic experience and overall, as I said in a previous post, I think you should do it. But just don't expect them to be fluent in such a short time and don't expect them to stay interested after you get back to the UK either.
I can feel your eagerness and enthusiasm coming through in your posts, and giving my children the opportunity to learn French "organically" was one of the selling points of us living here, so I get it. But it has been a longer and slower process than I imagined and everyone had told me that they would be fluent in 3 months. If my eldest (DD - 5) was doing 6 hours a day in school with no exposure to English during that time, I'm sure her French would be better than it is now. As it stands with part-time hours and two other English speakers in her class, plus the teacher who will speak in English if necessary, it hasn't been as quick as everyone said (Oh she'll be fluent in 3 months).
Talking to the teacher about it she says that she could separate the English speakers and she could speak only French in the classroom, but she feels that is a bit harsh for a newcomer who is settling in and getting used to all sorts of new things in a different country. Next year she will reduce the English she speaks (she keeps the same teacher) to DD as she feels she will have had a good grounding in the language and that she'll be that bit older and will cope with it more then. I am comfortable with this approach and rather than ramming French down DD's neck, she can pick it up at her own pace, along with adjusting to everything else a move abroad entails (missing friends, different food, different TV, no grandparents etc - ok slightly different if it's just for 6 months, but your children won't grasp the timescales...).
If there is the possibility to revert to English, this is what your kids will do. It's more natural to them and if they know it's possible to communicate in English, they will. Immersion is far more effective (but harsh) and would be a better way to get them to a fluent level in the 6 months that you have.
I also think that keeping it up at home afterwards will be a huge task.