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Hello! I am sorry to hear about this situation. Is there a case pending in court regarding the children? If so you would petition for guardianship there. If not you should talk with a guardianship lawyer about getting guardianship over the children and getting them into your custody. It sounds like these children should not be near mom or her boyfriend.
I wouldn't try and negotiate with DCFS or mom, I would move immediately to get a court order. Generally the courts will want to place the children with family before placing them in foster care if there is a suitable family member.