What is the procedure for allowing an immediate relative to come to the us with two minor children?

I want to file an I-130 petiton for my mother. I am a USC over 21. She has two minor children that she does not want to leave behind. I understand that no derivative beneficiaries are allowed under the immediate relative category but is there some sort of exception that allows my mother to bring her two children with her when she immigrates? What is the procedure and forms needed to achieve this?