Any time the goblins cross dress, which is a lot of the time.
Some of Jack Frost's more childish moments can count.
In the movie, there's this exchange.
Rachel: Oh, I know I should just ignore her. But Lydia is such a—
Rachel: ...I had another word in mind.
The movie has most of the three goblins' antics, especially when the fat one, Newton, is coerced into pedaling up a hill by way of dangling a carrot in front of his face.
Olympia the Games Fairy's book has Jack Frost winning an award for being a bad sport, which is a green cabbage. He's overjoyed.
Judge: But first, we have a very special prize to present. The prize is for 'Bad Sport of the Day!' And our winner is the runner who appeared at the end of the race!
The same book also has Jack Frost being unable to stop running because of the magic sneakers. The descriptions are funny enough, but it really becomes hilarious when the illustrations show him running laps in his castle.
Cheryl the Christmas Tree Fairy's book has this sign as Rachel and Kirsty see a pen of Santa's reindeer.
REINDEER BARN. PLEASE KEEP THIS DOOR SHUT. (AND NO EXTRA CARROTS FOR RUDOLPH. HE'S ON A DIET.)
Some of the illustrations in Lindsay the Luck Fairy's book, like Rachel running into a tree.
In Brianna the Tooth Fairy's book, a goblin hides in Kirsty's wardrobe and comes out wearing at least three dresses and many other clothes. As he escapes into the streets, a boy complains that an "old lady" stole his scooter.