Il est génial papy! (1987)

When 10-year-old Sebastien's mother doesn't pick him up for a holiday from boarding school, he runs away and invades the life of his grandfather, a down-and-out violinist who didn't know the boy existed. They, in turn, invade the wedding of the violinist's ex-wife, the boy's grandmother, who sends a cute babysitter to help out. A very cute French movie, now with English subtitles for the first time.