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 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. Even his wife (who clearly had no idea he got this for Ralphie and was visibly displeased about it), couldn't be too mad because of how excited they both were.
Taken Up to Eleven in the Musical when the Narrator says that this was his best Christmas gift ever, claiming first because it was his Old Man then correcting himself and saying that it was the best because it was his Dad who got it for him.
This line of dialogue says it all.
Old Man: I had one when I was 8 years old.
Although played for laughs, when the Old Man insists on taking his family out to the Chinese restaurant for Christmas dinner after the Bumpuses' dogs ruin their turkey, it shows that he is not gonna let anything ruin his family's Christmas.
At the Chinese restaurant, the staff do their best to sing Christmas carols, and even though the family can't keep from laughing at the staff's mangled English, everyone takes it in stride: the restaurant owner is also laughing at the bad singing, and it's clear the family appreciates the efforts the staff are making. As the narrator states, "That Christmas would live in our memories as the year we were introduced to Chinese turkey. And all was right with the world."
In the stage version, the Old Man is screaming and cursing up a storm after the Bumpus hounds make off with the turkey, understandably fuming. What finally gets him to stop? When he sees his wife standing in the kitchen, crying because all her hard work is ruined.
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."
Easy to miss for its subtlety, but there's a wonderful moment between Ralphie's Mom and Dad here. After downplaying the fight to Dad, Mom changes the subject, asking Dad about a story in the newspaper. Look carefully, and you'll see their eyes meet, and you can see the conversation they're having: Dad is saying, "I know you're not telling me everything. I don't mind that, but do I need to get involved?" and Mom is answering, "No, don't worry, I've got this." All in a split-second, without them opening their mouths. (Kudos to Melinda Dillon and Darren McGavin for nailing this scene.) They may have epic battles over the leg lamp or the Christmas tree, but when it comes to the big things, Mom and Dad are in sync and trust each other completely. Good Parents / Happily Married indeed!
Later, when Ralphie breaks his glasses, he cooks up a story about an icicle falling and hitting his face. His Mom buys it... or does she? The icicle story is pretty lame, when you think about it. It's more likely that Ralphie didn't fool his Mom at all, but she decided to let this one slide, knowing Ralphie had learned his gun safety lesson already, and didn't need her berating him. Mom knows what she's doing.
In the TV version, the narrator finally realizes he didn't fool his mom after all.
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."
Randy picking up Ralphie's glasses off the ground and holding onto them while Ralphie is beating on Scut Farcus, and later on hiding in the cupboard beneath the kitchen sink and crying because he's worried that the Old Man is going to kill Ralphie.
There's also him running to get their mom to break up the fight. And it wasn't the kind of "Ooh, you are SO busted!" It was more, "Oh, no. Ralphie might hurt himself, I gotta do something."
On a meta level, TBS's 24-hour marathon is just as much a celebration of the movie's impact on pop culture as it is a way for the channel to give as many of its employees Christmas Day off as they can.