the order of death as I see it is only correct on the assumption Sheila did it.
otherwise its highly improble
but of course it just happens to be the order of death that best suits the inheritance claims of the wife of the main prosecution witness.
why sneak past the adults to kill the kids when he can just kill the adults first.
I disagree with your conclusion. I can't see why the order of deaths of the children excludes either of the potential perpetrators.
The killer did not need to sneak past anybody, everybody was in bed.
The inheritance side of things doesn't make any sense either. The only relative in line to inherit was Pamela, she refused the inheritance which then filtered down to others. The 'others' couldn't possibly know that Pamela would refuse.