I doubt that the document you signed was a power of attorney (POA) with respect to your minor child since minors are incapable of contracting in all states in this country. Rather, I suspect the document was some guardianship document signed by yourself with respect to your mother giving your mother some authority to have the medical procedure done as to your child that you do not approve.In any event, I suggest that you consult with a family law attorney and have some document drawn up rescinding and cancelling whatever power you gave to your mother with respect to your child.