When the Old Man points out Ralphie's final gift tucked away in the den. No points for guessing what it is.
What makes this especially heartwarming for me is the fact that the Old Man is the only significant adult in Ralphie's life that the boy doesn't directly talk to about the BB gun.
And the Old Man, who tends to be either exasperated bordering on grumpy or enthusiastic to the point of eccentricity (with the occasional mixing of both to create Angrish in human form), can only giggle like a school girl when he sees his son open the present he wanted the most and was completely convinced he would never get. The fact that he gets so much visible joy from making his child happy is truly heartwarming.
This line of Dialog says it all.
Old Man: I had one when I was 8 years old.
Both Parker parents watching the snowfall while "Silent Night" plays on their radio. "Merry Christmas, sweetheart," coupled with a loving embrace.
Ralphie's mom comforting him after he beats up Scut Farcus, as well as playing down the story to his dad.
"From then on, things were different between me and my mother."
Ralphie's and Randy's mom comforting Randy after he gets scared by Santa.
Only in the stage show, a scene following Ralphie's beatdown of Scut Farcus between Esther Jane and Ralphie. Esther Jane gives him a card that she makes very clear she spent her own money on and picked out herself and rushes off in embarrassment. Ralph notes in his narration that the card is incredibly sappy and girly, and normally the sort of thing Ralphie wouldn't care for, but in this case, he didn't mind it. "Maybe even liked it a little bit."