From the JFTR
U10106 DEPENDENT SUPPORT
D. Legal Separation Agreement or Court Order Stating Support Amount. If there is a court order or legal separation agreement stating the support amount, a member must contribute to the dependents support the amount specified therein, but in no case may the support payments be less than the applicable BAH-DIFF rate.
1. When a member is divorced from a nonmember, and they share joint legal custody of a child, and the ex-spouse is awarded primary physical custody, then the member is a non-custodial parent for housing allowance purposes. If the members court-ordered child support is less than the applicable BAH-DIFF rate, and the member is not residing in, or assigned to, GOVT QTRS, the member is only authorized a housing allowance at the without-dependents rate. However, a member who pays additional support to the ex-spouse having primary custody of the child(ren) so that the total child support provided is equal to or more than the BAH-DIFF rate, and who is not assigned to GOVT QTRS, is authorized a housing allowance at the with-dependents rate.
From
http://www.defensetravel.dod.mil/site/faqbah.cfm#Q28
29. I am divorced with children, what is my BAH allowance?
It depends on whether or not you have legal and physical custody of your children, pay child support, and/or live in single-type government quarters. If you have legal and physical custody of your children, then you are authorized BAH at the with-dependent rate if not assigned adequate family-type government quarters. If your former spouse has custody and you are paying adequate child support (at least in an amount of your BAH-DIFF rate) you are authorized BAH at the with-dependent rate if not in government quarters or BAH-DIFF if assigned single-type government quarters.
From Army Regulation 60899
17. Entitlement to military allowances
c. Where a soldiers entitlement to BAHWITH or BAHDIFF is based solely on the financial support the soldier provides to a family member, the soldiers entitlement to this allowance is only authorized if the monthly financial support provided by the soldier is at least the amount of BAHDIFF. The Defense Finance and Accounting Service (DFAS) may cancel a soldiers entitlement to BAHWITH or BAHDIFF and recoup past payments of BAHWITH or BAHDIFF when a soldiers monthly financial support is less than BAHDIFF regardless of the current residence of the supported family member, or, if the supported family member is a child, regardless of the soldiers marital status when the child was born (see DOD 7000.14R, para 260406B).