How do I protect child who I have guardianship of?

My husband and I have spent the last 15 months fighting for guardianship of our 15 month old grandchild. At the beginning of this month, the judge ruled in our favor and we were given permanent guardianship. The child’s mother has significant mental health issues and it is unlikely that those issues will be able to be successfully treated in the future. We have had our grandchild since she was born. Since we now have permanent guardianship, it occurred to me that we now need to create a will and change life insurance beneficiaries, etc. If anything were to happen to us, we would like the child to go to our other grown child, who also agrees with this. How do we go about protecting our grandchild in the event of our demise? How can we keep her from going to her biological mother (with whom she has no relationship with) if something were to happen? Also, are there any others pertinent issues that haven’t occurred to me? Thanks so much!