Careful about the hot water bottle, as they don't hold heat for long, then they get cold, so you'll have to check them often.
Do you have a heating pad (electric) like the ones for aches and pains? I've put one of those under the pan of a cage (just only under half) so they could sit and the warmth would rise up to them. Warmth makes a big difference.
I'll keep my fingers crossed for you. There's nothing worse than a sick kid.