One suggestion I always have is take holidays in the target country and if possible send the child to a host family there (can use organisations like en famille

One suggestion I always have is take holidays in the target country and if possible send the child to a host family there (can use organisations like en famille). You don’t need a long immersion to learn the language at that age. I learned Spanish by staying 3 months in a host family (and going to school) when I was 12. A friend of mine did the same at 8. That initial short immersion combined with medias and occasional travel helps a lot and is more fun than classes would be.