Thanks to ScreenTerrier and DR.com we know 3 of the child actors from the Epiloge.  First, Ryan Turner was cast as Ron and Hermione’s youngest son, Hugo.  Next, Harry and Ginny’s son, James Sirius Potter, is played by Will Dunn.  Their other son, Albus Severus Potter, is played by Arthur Bowen.

Albus Severus and James Sirius may both be seen here and here.  Albus is with his mother and James is with the trolley.  The actor playing Hugo may be seen helow: