If knives are an issues then let's explore some other possibilities. An atheme is by no means an essential tool for a witch. Many witches will use a staff or a wand to do the job of the atheme. Many non wiccan witches don't bother with tools at all and use their hands to direct energies instead. Think about what you can substitute that is meaningful for you. And I strongly advise that you do not disregard your mother's wishes. Magic performed amid the worry that you will be discovered breaking the house rules won't be very effective at all. Better you use a wand or staff and work in the confidence that you are being honest with your parents.
Image links to Pagan Dreams where this and other athemes can be purchased.