Mandate strict house density rules

Your words, spoken in a low baritone that seems to vibrate the very floorboards, cut through their hushed awe. "The estate is large. Your forms are unstable. Effective immediately, we operate on a density protocol. No one strays more than fifty feet from me, at any time." You don't raise your voice. The sheer acoustic power of your chest does the work. They react as if you've slapped them, a collective, sharp intake of breath. Their brief, defiant bravado evaporates.
Kim is the first to break, her competitive spirit momentarily overriding the engine's imposed docility. "Fifty feet? That's... that's a restriction! We're not pets." Her argument is strong, but it's undermined as she speaks. Her right arm, the one she throws out for emphasis, dissolves into a pale, shimmering outline for a full second. The sight makes her swallow, her gaze darting to you, a flicker of genuine fear in her eyes. She needs you to be real. Without you, she is nothing but a ghost of a story.
Misty, ever the fiery tomboy, scoffs but doesn't dare move away. "You can't just keep us on a leash." She crosses her arms under her impossible chest, a gesture of defiance that only accentuates her top-heavy form. The motion seems to drain her energy, and for a terrifying moment, the pigtails on one side of her head flicker, the orange hairs becoming translucent, like smoke. The engine is a cruel thing. It gives them life but makes that life dependent on your proximity, on your mass, on you.
Daphne says nothing. She simply steps closer, until her tiny body is nearly pressed against your leg. She looks up at you, her violet eyes wide and pleading. The fear in her is different, not of the rules, but of what happens if those rules aren't followed. She understands the fundamental truth of this new existence. You aren't their master; you are their god. Your presence is gravity. Without it, they are adrift in a void, their very atoms threatening to unspool into nothingness.