Let's break this into two parts... kids and chickens.
KID: You, mother did nothing wrong. Nor did your child. The situation is terrible but your daughter likely left the chickens in the feed bin because your fat ladies were in there having a grand old time at the buffet and she knows to close the feed bin when you leave. Or possibly she just wanted to put them in the feed bin because it's food and what a better place for chickens! There's no chance of any ill will there- and like any parent you just have to do your best and sometimes things happens, no one is perfect. I wouldn't even mention the dead chicken to her, but it wouldn't hurt to say something simple like "and let's make sure there are no chickens in the feed bin when we leave or close it".
CHICKEN: It's incredible that your Aus. didn't make it, sorry to hear that- but I have to think somehow it was just her time. Judging that the BR did fine and given the relatively short period of time- something "odd" happened. Usually in the dark chickens will just hunker down, it's nature. Likely in this case the BR decided to go a bit nuts trying to get out (or both of them) and it escalated when they were running into each other in the dark and that there was unlevel ground (feed) moving under their feet - there was an accident of birds smashing into birds. No visible injury likely means some kind of blunt force issue- likely a neck break. Or, something just unknowable or pre-existing condition that manifested just then. I suppose it is possible that she suffocated too, but given that the BR just "jumped out" that doesn't seem likely to me because she would have been low on air at that point too and a single breath isn't going to magically revive her in 1 second. There's also the chance something happened before the chicken was in the bin and she was placed in there, maybe your daughter accidentally squeezed her too hard or grabbed her by the head, or was too rough putting the Aus in there or put the lid on too hard and hit her. But in any of those situation a 4-year old is too young to comprehend the magnitude of death in the situation and you should just try to teach her proper handling, etc. Again, really no way to know what happened so I wouldn't go overboard.
Either way- no one is at fault and sadly chickens die all the time. It's our job to do our best to keep them alive and happy for their short chicken lives and you, and your daughter, did all you could to do that. So all is good in Chicken Karma and your life. Sad, but move on and keep enjoying the rest of your flock!