D ir: Xavier Legrand. Cast: Léa Drucker, Denis Ménochet, Thomas Gioria, Mathilde Auneveux, Mathieu Saikaly, Florence Janas. 15 cert, 94 mins Directing child actors credibly is one of the hardest things to pull off. There are great filmmakers out there who have bungled it totally, or merely acquitted themselves. The French write….