can she speak French?
How long are you likely to be in France for?
My dd is 5.10 and so is in CP (one of the youngest). If your dd starts in CP she won't be that much younger than the others.
The French system is a bit out of sync with the British one at that age and in fact it is only in CP that they really start phonics etc. In GS they will be learning the alphabet, how to count to 30, how to use the numbers up to 10, and then they do masses and masses of work on hand writing, drawing loops and swirls etc.
My dd can read a very little bit (she can recognise simple words in French and English, and she has got the hang of putting sounds together to make words, but she hasn't learnt all the phonics by any means. I think she is at the top end of her class with regards to reading, and all the children in her class are able. (and she is in CP).
On the other hand because they start so late, they learn fairly quickly, so by Christmas all the children in the class should be reading fairly well.
So for reading, I think your dd would probably be better off in CP.
But then it all depends on other things too, like her general maturity (for coping in a class where all the children are older than her), her level of French, whether you are going to be back in Britain soon or whether you are staying in France long term, and the general ability of the other children in the school.
Bear in mind also that although the teacher may not expect the children to be able to read or spell, he may expect her to be able to do fancy French joined up writing.
hth