Les P'tits schtroumpfs (1988)
Directed by William Hanna

Film Synopsis

In the course of an experiment, Papa Smurf manages to break his hourglass and asks three of the other Smurfs to go and fetch him a replacement.  The enterprising Smurfs go to Father Time's house and find that he is not at home.  Undeterred, they climb in through an open window.  When Nat Smurf's butterfly disappears inside a grandfather clock, the three Smurfs follow him inside, but the door closes behind them.  The hands on the clock turn backwards and when the Smurfs leave the clock they find that they have all been rejuvenated, reduced to little Smurfs!