@bmpasternack this is not a tax issue. It's an estate structuring issue.
How much money are we talking about? is this a grandparent - parent - young adult situation?
First, the record keeping and cost to maintain is not simple or cheap. (i've personally looked into this)
a) Lawyers are required to set this all up (figure easily $2000 - $5000)
b) Who is going to be the Trustee? Is that going to be a 3rd party firm? if you anticipate this trust outliving A and B, who is going to manage this whole thing if not a third party (e.g. a trust firm) that charges for its services. Figure around 75 b.p. of the asset value (annually) to hire a trustee firm (assuming these are financial assets).
While you recognize the Trust will distribute to C over time, are you aware of the agressive nature of the tax rates for Trusts? At $13,000 of income, the tax bracket is 37%, so the Trustee is motivated to distribute an amount of money that exceeds the income of the Trust each year. The beneficiary pays the tax at his tax rate.
Another issue is whether the Roth distribution is a 5 year or 10 year requirement. My understanding is if the beneficiary of the Roth is a named person, it's 10 years, even if the owner is a Trust. But you introduced the notion that a Trust could be a beneficiary and I suspect (not sure) that suggests a 5 year payout requirement.
Controlling money from the grave is far from a free lunch! Suggest employing an estate lawyer - this is quite complex to discuss on an anonymous tax community board.
ps does this mean you read "Beyond the Grave"?
https://www.amazon.com/Beyond-Grave-Revised-Updated-Children/dp/0062336223
