Here's a couple:

Two different child actors played Tony Danza's childhood self and of course, Danza played a grandfather(I think) in Who's The Boss?

Nikki Cox played Young Iola in Mama's Family

Anymore?

Matthew Labourteux (sp?) from Little House on the Praire played Pa Ingalls as a child.

Maureen McCormick played little Endora in Bewitched.

Don't forget the late Johnathan Brandis as Young Sledge in Sledge Hammer...

Billy Mummy as Darrin Stephens in an episode of Bewitched.

Roseanne - The guy who played DJ played a young Roseanne, while Dan played an old(er) DJ

Sophia on Golden Girls played herself out of wig/old makeup as a younger mother (not really a child character but still played her younger self)

Who's The Boss - Candice Cameron played a young Mona (the episode where her brother opens a hotel)

Family Ties - Steven Keaton was shown in flashbacks, Alek/Mallorie were shown in a Christmas scene as kids, and also Alex when he was a baby