I'm so sorry to hear this happened. One would think a bird boarder would know better. I've had 2 bad experiences with pet sitters, but luckily none of my animals died from it. I just don't think I'll ever go on vacation again.
I just can't even imagine the stupidity of putting the 2 birds together like that. It's not the too's fault at all. Wow, I didn't let my 2 conures be together for a couple of years because I worried my Nanday might hurt my Blue Crown. My Blue Crown just didn't have it in her to fight back, and the Nanday was very jealous of her at first. I have a new Amazon but I'm not going to let him be out at the same time as my Nanday until I know it's ok. And if I don't know for sure that it will be ok, they won't be out together. And I would never put them in the same cage. The Nanday might be a tough little girl, but she weighs 1/3 what the Amazon does. And knowing the Nanday as well as I do, it wouldn't surprise me if she was the one who started an altercation.
I even put my cat in my bedroom when the birds are out. She doesn't know that she could hurt them. She wants to play with them. But, I know even a tiny scratch could kill one of them. And pouncing on them and biting them like she does with her toys (or me!) would certainly kill one.
As for the developmentally disabled child, he may not have understood what he was doing. He may have been trying to hold them and crushed them instead. Small children of normal intelligence can accidentally do things like that as well. They will hit a dog or cat when they are really trying to pet it, and must be taught how to properly interact with an animal. I used to have several barn cats that were terrified of small children. They knew the kids might handle them roughly and chase them as well. I had to teach them how to pay attention to a cat. However, the father is the responsible party there. He may not have noticed when the first couple went missing, but certainly after that he should have noticed and taken measures to protect the birds.
I'm so sorry to hear these stories of animals lost to stupidity and ignorance. They truly are our children.