In France, the educational programme is uniform throughout the country and pupils are able to change schools anywhere at any time without disadvantage. In fact, the French system is designed to be the same wherever it is throughout the world, right down to the timing of classes and course work from hour to hour during the day. This probably makes for the most seamless transitions of any other educational system, with the IB programmes the possible exception.
Transferring into the French system, however, can be overwhelming - in general the younger the child, the easier it is to adapt. The system is rigorous; if the child is new to the language, then tutoring in French would greatly assist the adaptation period.
On the other hand, leaving the French system and returning to the UK or to an American school should not present any educational problems. The child will normally be equipped with a good work ethic and a comparable academic level.