Your husband needs to have the letter done, and signed by the children's mother, preferably in the presence of a lawyer or notary. She must understand that if she blocks the process now, HE CAN NEVER SPONSOR THEM LATER. It is a very serious decision, and she needs to think about it carefully. If anything happens to her, their father will be barred FOREVER from bringing them to Canada to live with him. So make sure he explains this fully to her, and then if she is still set in that decision, he gets her to sign something in the presence of witnesses saying she understands and she has made that decision.