If slinkyboo is a native English speaker, I cannot see how her 6 and 4 year old would lose their English by going to a French school, rather than a Fr/Eng one. In our local francophone school, there are over 30 native English speakers, some speak English only at home, some speak English + French, some speak English + another language, some are born in Belgium to one Belgian parent, some are born abroad and came to Belgium between the ages of 2 and 11. If you met this group of 30+ children, you would not be able to figure out who speaks at home only English, English + French, English + other language. The common factor is that they all have a native English speaking parent and they speak it at home. What slinkyboo must NOT do is attempt to change the home language to French or speak a mixture of French and English, if it is not the case already, ie if the parent does not speak French as a mother tongue, refrain from the temptation to speak less than perfect French to your children. I can write this because I do exactly this with our 2 youngest and I know I shouldn't, but they insist on speaking French to me, it is hard not to continue in French with them, I am now making as much effort as possible to reply and continue in English.
slinkyboo, I'm sure your children will be fine, especially since they have the experience of a bilingual school behind them. I am only speaking from anecdotal evidence, but from what parents have reported to me about the few younger anglophone children I have met who have NOT settled well into French schools and have been moved into a different schooling system, the common factor has been their parents' lack of acceptance that the schooling methods/system are different. I know this is a sweeping statement, but parents' commitment to their children's advancement in French seems to be a major factor in their children integrating. Of course a school which welcomes non francophones and offers less boring teaching methods and lots of extra-curricular activities and parental involvement helps :->
Perhaps it is worth asking the school immediately for your 6 year old if any of the teachers are interested in private tutoring a couple of hours a week? If there is a school homework club where the teachers actually offer help, worth joining? If your French is not very fluent, I can see you might have difficulty communicating with teachers, ask the school immediately if they can introduce you to parents who speak French and English, they may turn out to be of great help in the first few weeks. Helping with homework at home at the beginning might be a big hurdle, even with almost fluent French, I will never forget our then 6 and 10 year olds coming home with homework they just could not do without my help, I ended up writing on their sheets whenever I had to translate things when they were able to complete questions without help, so the teacher would know what they could not understand. I made us of visual French dictionaries, even a Maths dictionary for the 10 year old as he was doing things I couldn't even remember how to do in English and had no idea of French terminology! After a few months, our children started doing their homework without help, how it should be, but hard work at first.
Outside school, I'm sure you'll find something sports/musical/arty to do in French. I highly recommend Scouts/Guides for your 6 year old -here in Belgium, youth movements are still very popular and it's a fabulous way to form close friendships outside school, our own 6 year old starts in September and is now getting excited at the prospect, hopefully in France youth movements are still strong too. If you are lucky, you might also be able to find activities in English. When we first moved to Belgium, I arranged one activity a week in English for our children - Rugby and Rainbows (Guides) - it has led to friendships in English outside of school, even though the children no longer do these activities.
I'm sure your children will do just fine. Positive attitude is half the battle. Don't expect miracles - yes some children take longer than others, a very minority start speaking their new language immediately and never compare your children to these ones, the vast majority take several months before they have enough receptive knowledge of their new language to be able to speak it. Remember babies don't speak at birth, they take a while listening to mum and dad before they make their first words, in under 8s, any new language acquisition follows the same sort of "osmosis" pattern, speak coming usually long after comprehension.