I'm wondering if Kyron became a mini-Kaine in TH's eyes. She figured out she couldn't do anything about her husband without making herself look bad or not get what she wanted out of it, maybe not enough money. Or maybe she was mad at herself for chickening out on the murder for hire. This whole time, the resentment for Kaine is building and building, and who is she taking care of and dealing with everyday besides baby K? Kyron. And maybe, as she's getting angrier and angrier, she's seeing Kaine more and more in Kyron. Not only is Kaine still not giving her enough attention, the little boy she is blaming more and more keeps acting just like him, in her eyes.
And then, this project comes around, and she sees them bond over it, and is furious. More attention away from herself. And she's the one has to lug it to school and such, and keep dealing with Kyron, by now mini-Kaine. What if Kyron did something so Kaine like that she just lost it? She realized she can't get rid of Kaine, but she can't stand to have this mini-Kaine in her face anymore. Kyron's finally done something so Kaine like that someone has to go, and it's not going to her. She can't stand taking care of a kid that reminds her so much of Kaine and that takes Kaine's attention away from her, and also keeps a woman she can't stand, Desiree, in her life.
She starts thinking that without Kyron, her life would be better. Kaine would pay more attention to her. Desiree would be gone. It would just be TH, KH, and baby K (and even J. Maybe she blames Kyron for making J go away too). And when she sees Kyron acting like Kaine, and getting more attention from Kaine, it solidifies her plan even more, justifies it even more in her mind.
What I am saying is, I don't think this is something she came up with overnight. I think there was a rage that was seething below the surface for a long time, at least six months after the MFH plot. And she finally convinced herself that she just couldn't take it anymore, and if she couldn't get rid of Kaine, at least she could get rid of mini-Kaine, Kyron. At least he is someone she has a lot more control over her and is a lot smaller and easier to hide. And heck, while she's at it, why not let the school and a pedophile take the blame for it? She won't be suspected, and she finally gets what she wants. She can then divorce Kaine and get half of everything, or find a way to off him and get his estate.
I also think that planning is a lot different than carrying something out. I think she built herself up in her mind and thought it would be easy. Then she actually did it, and found it it's not as easy as she thought it would be, and she's not as brilliant or brave as she thought she was. And Le is a lot smarter and on to her than she ever thought they'd be. I think the only thing that has gone her way is that Kyron hasn't been found yet.