This small ski resort nearby Grenoble operated by the Alpe d'Huez ski operator is the paradise for beginners and children : slopes are large, not difficult and there is no risk of collision because almost nobody skis there.
For advanced children, you can even let them ski on their own without fearing to lose them.
Also, it is a good spot for off-piste skiing because the powder remains clean for several days and there is a lot of space around the slopes (unlike bigger resorts where every spot is ravaged in a couple of hours).