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.)
In Leona the Unicorn Fairy's book, a goblin falls into a pile of horse manure.
Lizzie the Sweet Treats Fairy's book has the goblins' cake, which is a bunch of cupcakes stuck together in a blob of green icing.
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.
Jack Frost interviewing himself in Alexa the Fashion Reporter Fairy's book.
Jack Frost's evil plan in the Sweet/Sugar & Spice Fairies series is to build himself a candy castle.
In Esme the Ice Cream Fairy's book, Jack Frost wants to build a swimming pool filled with ice cream. Esme decides that's too wasteful and gives him a bathtub full of it instead.
In Coco the Cupcake Fairy's book, Jack Frost tells a baker about his plans to build a throne of cupcakes. When the baker says that sitting on them would squish them, Jack Frost is offended and asks if he's saying he's fat.
Bailey the Babysitter Fairy's book is full of funny moments, as the goblins are terrified of human and fairy babies and run around screaming when they appear.
In Natalie the Christmas Stocking Fairy's book, the goblins each take a lick of the charmed candy cane and hate its sweet taste. Jack Frost is defeated when he tries to eat it, gags at the taste, and throws it away to where the girls can reach it.