I assume you're British, OP, and that your French husband doesn't have an issue with this trip? Was the trip that upset your 6yo also with this French school?
It's a tricky one, as French schools and attitude to children's upbringing is remarkably different from the UK.
There is a lot to be said for the benefits of the cadre (in this case through shared residential trip). The French rely on it heavily as a social compact, with child-rearing as much as anything else. They "do" individuality but often seem far too insistent about conformity. (This is a generalisation and not intended to insult anyone; I am part French).
The benefits can be manyautonomy, adventure, huge opportunities for learning... I'm not expressing it well but it's a source of amusement that the same French mother who will allow her 3yo on an overnight trip will also be ironing his "favourite t-shirt" t-shirt! -- when he returns to his provincial home each weekend from his job in finance in Paris. (I'm looking at you, Tata Marie-André.)
The point is I don't think sending children on these trips proves a lack of caring or protective instinct from the French parents. They will be struggling, and some of their children will be feeling anxious as well. It's just very, very different because they have experience to tell them it can be wonderful, in the end, and best for the children.
You're going to get many "no way" responses here because it's virtually unheard of in the UK. Please remember that all the other children going on this trip will probably turn out quite alright--if anyone can do it well, the French nursery/school system can.
That said, it's quite obviously your decision to make on behalf of your own children, and cottage visit for the family sounds lovely.
Best of luck.