Child Custody and auxiliary benefits

I have recently divorced my wife, who is on SSDI. We have one child that receives the auxiliary benefit, I have custody of him 72% of the time.

The Family Law Judge did not understand the auxiliary benefit and awarded her the full auxiliary benefit as being a part of her SSDI benefit. I went to the SSA office and showed them my custody agreement as was elected as the Representative Payee for my son.

Unfortunately my ex-wife also suffers from bi-polar (documented) and is prone to spending sprees and mis-use of the funds.

I am looking for the statues that define the criteria in the SSA making their determination on who is the custodial parent. I was told in the office it is the parent with the child 50% of the time. I would like to find supporting documentation for custody as well any information on her bi-polar potentially disqualifying her as the Representative Payee.

Again the SSA has already made me the Representative Payee I just need to find the supporting documentation to show the Family Law Judge.

Thank you,