A French film probably 1940's/50's. Deals with 2 children evacuated from Paris in 1940 when the Nazi's invade, can anyone help I saw it on TV 15 years ago?

It could be "un sac de billes", it was released in 1975.

It sounds like "Au Revoir Les Enfants" which is about a French boarding school that hides a little Jewish boy from the Nazis, he goes by the name of Jean Bonnet (his real name is Jean Kippelstein). Excellent movie. But it was made in 1987 so I'm not sure it's what you're looking for, but you could give it a shot Au Revoir Les Enfants