it looks to me that hand holding is moving, there seems to be a little blurring that suggests movement, so i wonder if the girl on the right has just blinked.
it was one pic and it had to be right in those days, we've all got pics of our kids pulling hideous faces, this might just be another
i agree the stance is odd, and the leg reminded me of calipers you used to see on kids with walking issues, i wonder if she was disabled in some way.
I'm wondering why you would consider she was dead. Under what circumstances would a child be asked to prop up and be photographed with another dead child?
Have you been able to see birth certificates? and depending where she lived census etc, they often record any disability. Just last week i found a census that records an ancestor as deaf.