Oh, I agree, the other reason was keeping cashing those cheques. It was always about money with the kids, money and revenge. If it was just that she was sick of parenting, she would have sent Tylee to college like she said she had, and sent JJ to Kay and Larry. But without the bucks from murdering Charles, she needed an alternate revenue stream, so, the money, what there was of it, was from the kids. But rather than keeping the kids around, it was more efficient to kill them and only have to spend it on themselves.
I think killing the kids was always a part of the plan, but it certainly took on more immediacy once they realised the money from killing Charles was never arriving. It was their undoing. If nobody had been looking for the kids, I really think they would have got away with Charles and Tammy. But those kids had too many people looking out for them, asking too many questions, until eventually, it unravelled the whole mess.
And there was nothing ambiguous about Tylee and JJ's deaths, how they were found, unlike Tammy or Charles. Nothing at all.
MOO