I can tell you why I bet, and I'm so very, very sorry to tell you this but I'm so angry right now I can hardly breath...It's the crushed walnut substrate at the bottom of the cage! First off birds don't need a substrate at all, for future reference, and I'm so sorry this happened this way. I own a bearded dragon and have had many reptiles in addition to birds. I can probably name 20+ people from my bearded dragon forum that have lost a young baby suddenly and they all used that damn crushed walnut substrate. I know many, more than 10, that had a necropsy done, and sure enough the lizard had the crushed walnut shells in either it's stomach or intestines, had internal bleeding, severe internal injuries from it shredding it's insides, etc. It's actually crushed walnut shells, not walnuts, so think about that...Broken up walnut shells, they are hard and extremely sharp. They accidentally eat a few pieces, go to sleep, and bleed to death internally, become severely impacted, etc. With the reptiles it's typically accidentally ingested when they lunge to eat a moving insect, but when it's a bird that dies from this stuff it's because they are walnut shells, they smell like walnuts, birds will pick at the crushed walnut shells, and it only takes a small amount to kill them quickly, or it builds up over days to weeks and eventually impacts them or slowly kills them from the bleeding...There are several petitions to stop pet stores like Petco and PetSmart from selling the stuff, it literally has killed thousands of pets.
I'm so, so sorry this has happened to you, but once again just for future reference birds don't need any substrate at the bottom of their cages at all, just newspaper...
As far as the other birds you've lost its a horrible thing to go through, losing 4 green cheeks that were a year old or younger in 5 years is not typical. Accidents happen, and if this was in fact the walnut shells substrate that certainly wasn't your fault. It's always good to take a look back and try to make sure you recognize why this has happened each time and take measures so it doesn't happen again. Birds do get sick and it's hard to tell because they hide it well, but having a certified avian vet and an emergency avian vet/hospital plan for 24 hours is a must.
I can't say much else except my heart goes out to you and especially to your son. If you do get a necropsy done please let me know if it was the crushed walnut shells, I'm guessing it was only because it has become so common I'm actually used to reading about it. It needs to stop being manufactured.
"Dance like nobody's watching..."