“They can do the work. They just won’t start unless I’m sitting right there.”
That looks like dependence. It’s actually a brain borrowing what it can’t yet generate alone: the steadiness to begin. Starting next to a calm person costs less than starting alone, which is why they work when you’re nearby and stall when you leave. This is support their brain can use, and it’s temporary. You’re the scaffolding, and scaffolding comes down as the skill goes up.
Body doubling is presence and only presence: you nearby, doing your own real work, available if they ask and otherwise out of it.