A china pig was a bed heating device rather like a stone jug with a screw-in stopper. It was made from white china clay (wally). Although filled with boiling water, grannies generally knitted thick woolen covers for these so that you didn't burn your feet on it. Thus insulated, and containing more water that the more modern (rubber) hot water bottle, it retained heat all night.

English translation:

It was made out of china and it weighed half a ton
But it will still keep you roasting when a rubber one is done
Just fill (it) with hot water and screw down the lid
And jump into bed with your wee china pig.