I really think you need to let the children decide on there own if they want to call her mom. Yes, she has been there however these kids my still be hiding feelings from there real mother's death. Tell the kids that you understand that they are having a hard time with this and that whatever they decide to call her name or by mom it is fine with you. They will come around and someday call her mom. You just need to wait until the time is right for them. I hope this helps.