Who should get custody when a parent pes? The mother, father, or the step mother, or step father?

I have been married for almost three years. My husband has legal custody of his 2 children 9, and 6. We have raised these kids in our household for the last 5 1/2 years. I helped him while we were going to court to get custody, and my name is on some of the paper work, as being involved with raising the kids, but not as a part of seeking custody. Neither one of the baby mama's send the court ordered child support, they would rather go to jail. Now as far as I am concerned these are my kids, and if something were to happen to my husband, would I have to return my kids back to their mothers. We haven't wrote a will yet, and he wants the kids to stay with me. I was told we could just write up a doent and have it notarized, stating his wishes for the kids, and send a certified copy to the kids mother's. Is this true. The laws differ in each state, and we live in tennessee. I spoke with an attorney over the phone, and he wanted 400.00 for a will that will state my husbands wishes. Is it necessary to pay 400 bucks to set this up. The baby mama's just want the money, because he is a vet, and also has been employed by the gov. for over 15 years. He is 50 and I am 38. I need good advice