I don't know with any authority what this song is trying to say, but I will give you an alternate interpretation that might make your relative rest easier.

I am picturing a young girl of the house in charge of minding the babies, keeping them in the shade beside the field where the rest of the family is working, in sight of mother but out of the sun. 'everybody's gone in the cotton and the corn' means everybody is picking or hoeing cotton. If it's really hot, which i guarantee you it is if they are hoeing cotton, maybe she lays the baby or toddler's pallet on top of an old grave slab in the little family cemetery, which many an old farm has right next to the fields. 'come lay your bones on the alabaster stones'.

There now, isn't that better?