I always put it in the notes, and its usually fine. Worse that happens is that the note isn't read until you check-in, but usually someone sees it before that and arranges it.

In your case, the reason you can't book a King + a child is because the room isn't big enough to take a king-sized bed and a childs rollaway bed. A double bed is smaller, so they can squeeze in the childs bed. This is irrelevant to your situation, though.