The repeated advice about not getting yourself locked into a wardrobe. It comes up often enough to make you wonder if it's something Lewis' goddaughter, his primary intended audience, made a habit of.
The meeting between Lucy and Mr Tumnus. Not having seen a human before, he asks if she's some kind of beardless dwarf. When she tells him she's in fact a girl, she then adds "and actually, I'm tallest in my class" with a brilliant "so there" expression on her face.
She suggests a handshake. Mr Tumnus doesn't know what that is and asks why he's supposed to shake. Lucy's eyes widen and she says "I don't know!" It's completely Adorkable.
The lioness◊ with the glasses and mustache at the end of the first film.
Also becomes a heartwarming moment if you realise it's a Call Back: earlier in the film, Edmund drew glasses and a mustache onto a lioness turned into stone by the witch. This one must be the same one, and she apparently doesn't hold a grudge against him for it.
When Mrs. Beaver is packing food before the group try to flee from the wolves:
Peter: What's she doing?
Mrs. Beaver: Oh, you'll be thanking me later. It's a long journey, and Beaver gets pretty cranky when he's hungry.
Mr. Beaver: I'm cranky now!
After they've packed most of the food supplies:
Susan: Should we bring jam?
Peter: Only if the witch serves toast!
(a little later)
Mrs. Beaver: You didn't think to bring a map?
Mr. Beaver: There wasn't room next to the jam!
Mr. Beaver's eagerness to get the kids to safety starts to wear on Peter.
Peter: (glaring at Mr. Beaver): If he tells us to hurry up one more time, I'm going to turn him into a big, fluffy hat!
Later, when the kids first meet Aslan.
Aslan: Yes, Peter Pevensie, formerly of Finchley. Beaver also tells me you plan on turning him into a hat.
When getting their Christmas presents:
Father Christmas: Lucy, Eve's Daughter. These are for you. The juice of the fire flower. If you, or one of your friends are wounded, one drop of this cordial will restore them. And though I do not expect you to use it, this [knife]. Lucy: Well, I think I could be brave enough. Father Christmas: I'm sure you could. But battles are ugly affairs. (turns to Susan) Eve's Daughter, Susan. Trust in this bow, for it does not easily miss. Susan: ... What happened to "battles are ugly affairs"?
After Father Christmas bids them farewell, Lucy turns to Susan with an unbelievably smug grin and remarks "Told you he was real." On the commentary, they mention the line was originally when he arrived but after he left played funny.
When fleeing with Mr. Beaver, they are to cross a wide, foaming, turbulent river:
Lucy: Don't beavers make dams?
Mr. Beaver: I'm not that fast, dear!
The scene at the end, when the children all fall out of the wardrobe and the Professor hands them their ball back.
A blooper of this scene features William (Peter) missing his cue to catch it, resulting in it clocking Skander (Edmund) in the head. The actor playing the Professor proceeds to laugh as Skander complains "Ow... ow..."
Susan and Lucy make note of the Narnians staring.
Susan: Why are they all staring at us?
Lucy: Maybe they think you look funny.
Susan's idea of a rainy day game is looking up words in the dictionary and guessing their definition. Lucy suggests a game of hide and seek.
For the filming of the Turkish Delight scene, the prop masters asked the director if he wanted the "fake" candy. He said yes...and they brought a fiberglass and wire piece which couldn't be eaten.
The pottymouth bucket that Georgie (Lucy) carried around. The boys claim she made up curse words and she references an incident (and clarifying "It wasn't you actually, Skander") where someone swore and her head whipped around to force them to pay up.
Skander (the actor who plays Edmund) is mentioned as not being permitted to eat sugar so William (Peter) apparently tricked him into eating sugar glass and is heard unrepentantly declaring, "And he actually went and ate silicon."
Skander's complaints that Andrew (the director) made him stand on the edge of a cliff holding a huge, heavyweight sword... while knowing he was scared of heights. Andrew is unrepentant.
The already funny moment of Peter forcing Edmund to wear a girl's coat is made better when Skander admits he actually loved his big, fluffy coat and had to tone his happiness down.
Georgie: It's a pregnant woman's coat, you look like a pregnant woman!
Skandar: No, I don't, I look like a gangster. I look cool.
Andrew recounting William's reaction watching his battle scene.
Andrew: I remember Will looking over at his girlfriend smiling, like he was saying, "Hey, I'm an action hero!"
Will: Andrew, you weren't meant to reveal that!
Meanwhile, there's Skander's lack of action hero-ness in the battle.
Skander: OK now, everyone look at me, not Will!
Will: Basically Skandar's not really a warrior: Him and Mr. Beaver have to go up and hide.
Skander: I'm amazing! Shut up, Will let me have one moment! ONE MOMENT!