Young anglophones in French-only schools
We moved here when our children were 6 and 10 to a Flemish speaking area. The children were put into a local school - it was a small village, so no chance of finding expats there. Although they found the language difficult at first, they easily made friends and within 3 months were conversing quite happily.
Academically they had few problems as they were given a lot of support within the school. At our request our eldest child repeated a year - year 5 - as we felt moving too quickly would hold him back.
They are now grown, have good jobs and speak 4 languages with ease.
Children are able to adapt much easier than adults so I think you won't need to worry too much. Just try and find a school that offers good support and one you feel comfortable in.