Okay, so you had a 401(k) and converted it into a Roth and, when you did so, you paid tax on the amount converted, right? At that point, as long as the money stays in the Roth, you shouldn't have to pay a portion of it for child support. If, however, you take any money out of the Roth, THEN you MAY have to pay support on such withdrawals.
Certainly the portion of the IRA withdrawal that is from interest or capital gains IS income and you must pay support on THAT. The circuits differs, however, on whether you have to pay on the withdrawal of the principal.