Omar Sharif’s son Tarek played a younger version of Yuri Zhivago in Doctor Zhivago. (He looked a lot like his father!)
And though she didn’t star with one of her offspring, Geraldine Chaplin played her own grandmother in a movie about Charlie Chaplin.
What if parent and child play the same person at different ages?
Morgan Freeman’s son Alfonso appears in Shawshank Redemption as young Red (in the parole room photos).
Jerry and Ben Stiller appeared as father and son in Hot Pursuit.
Phil Magera, April Magera and their son Bam in Jackass: The Movie 
Kevin Smith’s daughter Harley Quinn Smith played the baby Silent Bob in Jay and Silent Bob Strike Back.