In reply to my own question,since no-one else has, I have happened on one of the movies I was looking for.
'The Proud Rebel', directed by Michael Curtiz, starred Alan Ladd and his son David as father and mute son. The boy finds his voice to shout a warning to his dad at the end, saving him from being shot by Harry Dean Stanton, no less.
I just came upon it in a book, so a big thank you to me.
I am still hoping for an answer for the other one.