The best, XXXXX XXXXX and safest way to euthanize the cat is to have the vet do it. Most vets will administer the meds and allow her to sit with her cat, quietly and alone. You don't want to take any chances of giving the cat the wrong dose, and giving human medication, if she truly loves her baby, it's best to let a professional take care of it.
No one here is going to answer that question.
It would be not only inhumane, but inhuman.