I think you are mistaken. FAR from being an absent parent, it seems that Colin had been their main carer, supported, I believe, by his mother. It was with him they were living at the time of the party. NONE, but some of those here who wish to find a "child cruelty" stick to beat Sheila with, have found her guilty of such.
As for Colin's feelings about letting the children accompany Sheila to the farm, it was more a case of Sheila accompanying the children to the farm because the Bambers had requested a visit before the boys went on holiday with their father. It's true that he may have seen himself as being between a rock and a hard place but on balance he probably thought they'd come to no harm with their grandparents. I fail to see why you think there to have been "an expectation of trouble" on this particular occasion, more than any other. Surely, had this been the case, he'd have informed the Bambers of any concerns he had re Sheila. As it was, she appeared no more than withdrawn and displaying classic signs of depression. A change of scenery MAY have been better for her than being on her own and it was going to be for less than a week. Whatever his concerns re Sheila's illness and June's religiosity, he neither mentioned it NOR gave Neville the letter he'd written.
You again refer to what you see as a volatile situation full of tensions and resentments, and I CAN see why Sheila, having met Christine, might feel this way about June but if June and Neville didn't know anything of her meeting with Christine there would be no need for them to have such feelings. As I've previously said, everything about Sheila's behaviour suggests her being too devoid of emotional energy to kill, let alone employing a method totally alien to her.