The two young ponies that Pinkie Pie kidnaps during her Smile song were sacrifices. That’s why they were sad and not in school, that’s also why the one pony smiled as she witnessed the kidnapping. You see, Pinkie is a deity incarnation. We witnessed a few of her powers when she was stalking Cranky, like; completely erasing a statue of Celestia from existence, growing multiple limbs, squeezing into unreasonable spaces, summoning a parade (which may have just been fear induced obedience to her whims), and turning herself into a firework. I believe that were she to go for longer than a month without a child-sacrifice the powers she has would burst free of her form and plague Equestria with no hope. It had been a month and the suppression of her powers hadn’t fully taken effect until after the conclusion of the episode.
Why a child sacrifice?
The innocence, silliness, and energy is almost opposite to what god she is hiding inside of her. This is why she has so much of these qualities and the story about her cutie mark, her family, and the rock farm are all fabrications. The entire population of Ponyville, save for Spike and Twilight, knows the truth. There are three others who know.
Star Swirl, or at least he did know if he has died.
Celestia, she was an accomplice since she’s not beyond this level of trickery.
Another who attempted to bring this deity into Equestria. I’m keeping that one as a secret for now, it’s no fun if you spill the entire story.