The bankruptcy code expressly deems child support as a nondischargeable debt. Bankr. Code § 523(a)(5). In practice, this provides state family court judges with effectively unlimited discretion as to the determination of whether or not a particular business expense may be deemed ordinary and necessary for the production of income, as would be the case under Int. Rev. Code § 162.
Simply put, the fact that an expense is a reasonable expense under federal bankruptcy or tax law, may be useful evidence in family court. However, the final judgment on any particular expense is entirely within the discretion of the family court judge.
The typical example is a leased luxury vehicle used by a real estate agent. The vehicle may be ordinary and necessary to the agent's production of income, but the family court can determine that the agent uses the vehicle for personal use, and that the agent could use a C300 Mercedes, just as well as a CLS550 -- consequently, half of the lease payment shall be deemed disposable income for the purposes of calculating child support. And, as soon as the current lease expires, the court will expect the agent will lease a less costly vehicle, in order to maximize child support payments.
Note: I personally find child support laws to be astoundingly inequitable in general. However, they "is what they is," and it is extremely difficult to win these battles in court. Consequently, a child support obligor
must be prepared to "downsize" their lifestyle.
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