It is worthwhile pointing out that giving cash payments to an ex-wife for “child support” is a concept stemming from non-Jewish legal systems. Halacha has a vastly different set of obligations for supporting ones child than does the non-Jewish courts. Most of the items and charges secular courts charge a parent to pay for child support, are not legal obligations for a parent to pay for, under halacha. Furthermore, under halacha a parents obligation to support ends when the child reaches his 13th birthday, and according to some shittos, even his 6th birthday.
Of course, the parent ought to feel a basic moral obligation to support his child. But such a moral imperitive is not necessarily legally enforceable under halacha. And such a moral imperitive certainly does not extend to supporting an upper-class lifestyle for the child or ex-spouse. And it also should not be expected to be more than the spouse can afford based on his income level. Even if a secular court orders more support payments than affordable.
So, if an ex-spouse requests or accepts such payments, against halacha, they are essentially engaged in thievery al pi din. Especially if they use any (even a small) portion of such payments for their own benefit rather than their child’s benefit.