We used to have a 40 gallon drum submerged in the earth outside of the barn with the lid on it for disposing of piglets that suffered this fate. The seething mass of maggots in there made them disappear quite quickly. It wasn’t economically viable to grind them up and use the protein for another purpose. It’s sad but it does happen. Even with specially devised pens that gave the piglets some safe margins away from the roll radius of the sow, they often seemed to not know that their young were even under them.