If you see a very full crop and are worried, wait a few hours and look again, it should look better. The parents could either be under nourished themselves and are unable to feed the chick, or the parents could just be inexperienced. I suggest more food for starters. Whatever they will eat. And if they are picky eaters, add vitamin drops and other supplements to their food and water.
All new parents have to learn sometime and sometimes it does cost the lives of a few clutches.
Tho if you are determined not to loose these chicks you can always feed them yourself. Unless some major trauma befalls the little chick, the crop will not pop when it gets full. Chicks that are still bald and very well fed have huge gorged crops. It actually is not uncommon. After the crop fills to capacity the chick will no longer be able to lift its own head.