How we cite our quotes: (Chapter.Paragraph)
Quote #1
Seth resurfaced at the edge of the pool and threw his arms up onto the flagstones, trying to drag himself out of the water. Kendra stooped to assist him but shrieked instead. One arm was broad, flat, and rubbery. No elbow, no hand. A flipper coated in human skin. The other was long and boneless, a fleshy tentacle with limp fingers at the end. (8.57)
Kendra's right: this is definitely an ew moment in the book. When Seth becomes a mutant-walrus-thingie it's gross to look at, and probably not very fun from his perspective either. It must feel really weird (if not also painful) to have your body rapidly transform itself into shapes it's not meant to occupy.
Quote #2
"Seth was altered by magic imposed upon him. But the potential to fall and become an imp is a fundamental aspect of being a fairy […] Muriel might be able to undo the enchantments forced upon Seth. Reversing the fall of a fairy would be far beyond her capacity." (8.130)
According to Grandpa, there are magical rules about transformations and how they work, and for fairies, the possibility of becoming an imp is always there—it's just part of who they are. Since this is so ingrained in their magical identities, you can't just undo it with a spell. What happened to Seth, on the other hand, was imposed from the outside (by super-annoyed fairies), and so in theory a spell could reverse it.
Quote #3
The baby dove into the room, transforming grotesquely as it landed on the floor in a deft somersault. The child was replaced by a leering goblin with yellow slits for eyes, a puckered nose, and a face like a dried cantaloupe. (10.138)
This kind of rapid transformation, from cute to gross in 0.3 seconds, must be the work of some kind of magic. An illusion spell? An actual bodily transformation? We're not sure, but either way, we don't like it.