Hi I'm sorry for your losses. I know how heart breaking that can be.
Getting a necropsy done on the next one to die should give you a definitive answer but if their poop looks healthy it is unlikely to be coccidiosis and probably not worms, but worth getting a poop sample tested. Any vets can do it and they shouldn't need to examine the bird. Here in the UK there are independent labs that you can buy a sample kit online and send the sample off in the post and sometimes even get next day results and it is cheaper that the vets....it is only £9 I think. I'm sure there must be such a service in the USA too.
My guess would probably be that they are suffering an outbreak of Marek's disease. The two commonest killers of young birds are coccidiosis and Marek's. They are both very widespread and easily contracted. Unfortunately there is no recognised treatment for Marek's. Sometimes it causes paralysis or an inability to coordinate limbs, and wasting away despite eating well is another symptom, but birds can also die suddenly from tumours on their internal organs without showing any of the classic symptoms. It is a very difficult disease to diagnose if they don't exhibit the "classic" symptoms.
Your state agricultural dept should be able to advise you where you can get a necropsy done and how much it will cost, or you could try to do one yourself if you are not squeamish. Opening up the abdominal cavity and inspecting all the organs can often give a good indication of the problem in my experience. There are a few threads here on BYC where we share photos of our DIY necropsies and discuss opinions and ideas on possible diagnosis.
I hope you are able to get to the bottom of the problem or that it settles down on it's own soon.
Best wishes
Barbara