Marsha and her brothers have a reputation for being rambunctious and destructive. Given they are werewolves, this makes more sense that they have few outlets for handling all the excess energy.
Why doesn't the enchanted hot chocolate hurt the kids? Their bodies are used to changing every full moon, and the transformation negated the effects.
"Santa's Helpers"
The elves putting the kids in a sack is similar to what Krampus does in the European tradition of Christmas. He takes away bad children in his sack if dragging them to hell, or gives them coal and switches. The reader can assess that Spenser and Beth have been very naughty with how they bully Diana. Being mistaken for elves is the equivalent to being Dragged Off to Hell during Christmastime, as we see.
Diana said that Santa punishes naughty kids after the twins refuse to sled with her after making her pull them up. Beth and Spenser scoff, saying Santa isn't real Perhaps that's why the elves showed up to kidnap Spenser and Beth, and Santa was practicing Obfuscating Stupidity when mistaking them for elves. The elves, after all, seem too smug about having supposedly caught some runaways, and everyone's name shows up on the Naughty or Nice list if you believe most movie canons. After all, a lump of coal may not be enough for twin bullies who don't believe in the Big Man to change their ways. It's a Cool and Unusual Punishment that Santa shows Spenser and Beth he is real, and they have to rely on Diana to identify them to the elves. Diana refuses to help them, adding the final nail in the coffin.
Beth and Spenser get kidnapped when they stay out after dark. All of the other kids go in, and they proceed to declare they're on Kings Of the Mountain. Soon, however, they wipe out and crash into a tree. Then the elves catch them in a sack. Now how does that saying go? "Pride goeth before a fall."
Much of the story has Beth and Spenser teasing Diana that she's not their sister because she looks different. Their mother gets annoyed and tells them to stop. As Troy Steele noted, it may have been a G-rated version of the twins invoking Cheating with the Milkman, which means their mother may have been as upset about the implications as she was about seeing Diana angry.
There are hints that Diana, Spenser, and Beth have a mom who spoils them rotten. She gives them two snowsuits brand new (which they don't appreciate), lets them stay out late sledding, and doesn't punish them for bullying Diana. (Rather, they sled away before their mother can ground them.) Diana is more mellow because her mom has stricter rules for her as the little sister, meaning a shorter curfew.
Given how casual they are about it, Marsha, Ricky, and Ronnie are fine with eating a human being if the latter tried to kill them. They're completely cognizant of their surroundings when turned into werewolves.
Mrs. Spooner has an entire pot of that hot chocolate. How many kids or neighbors has she poisoned for crossing her?
"Santa's Helpers":
Spenser and Beth's mother is going to find out her two older kids never returned from sledding after the sun had gone down. They won't be able to see her for five years, and that's if they are "hard workers" for Santa's workshop. The cops will also laugh at Diana if she confesses that she let Santa take them to the North Pole by pointing out they always said she was not their sister.
How many elves have run away from Santa's unfair labor conditions? We only know enough have for elves hunting down the fugitives, and mistaking two children for them.
On that note, how many kids wearing the red snowsuit with green trimming were mistaken for elves? Spenser and Beth may not be the only ones.
Diana goes Beware the Cute Ones and lets Santa's elves take away her siblings, even if she doesn't know they'll be condemned to eighteen-hour shifts for five years. Will she ever regret that decision down the line?
When the mother finds out where Spenser and Beth went and what Diana did, how will their relationship go?
