The Solo 401k has several advantages that an IRA cannot offer such as participant loans (allowing you to borrow from your retirement account for any legal reason including use in your real estate investing), no custodial requirement, no need for the additional expense and administration of an LLC in order to have that aforementioned checkbook control, about 10x higher contribution limits, additional tax benefits, etc.

Their aunt now has legal custody of the girl.  

Does anyone know the steps for filing a quitclaim deed or gaining custody of the property?

If you have IRA or can transfer money from your 401k, you may want  to set up Solo 401k or 401k custodial account (Sterling Trust and Guidant Financial are pretty much your only choices as far as I know) - both are vehicles for investing your retirement funds in real estate without paying taxes on it while it is in the account. 

This is actually my goal in life, but being a single dad with custody of my 2 kids, getting that $150k in the bank to start with is much easier said than done. 

just curious. also for boarding the loan you might want to look into Richmond-Monroe for custodial serives, doc prep for AOMs and Allonges, file audits, and missing docs in collateral file, Richmond is better and FCI.

The difference is the convenience and the cost, with IRA LLC you have the ability to write the check and pay to the repair man or utilities bill, with custodial SD IRA you have to request your custodian to do that, and each time that happens there is a fee that custodian charges. 

I would be grateful for some counsel re Custodial IRAs. 

I'd pay particular attention to experiences reported by those with custodial control versus those with checkbook control.

While you still have to follow the rules of SDIRA in terms of prohibited transactions, self-dealing, etc, it has many more advantages:Compared to an IRA, Solo 401k contributions limits are roughly ten times higher.There is no custodial requirement for the 401k.You don't need the additional expense and administration of an LLC to have checkbook control.There is a built in-Roth component whereas IRAs are either traditional or Roth, not both.A spouse can also participate in the same Solo 401k plan.The Solo 401k has additional tax benefits over an IRA when investing into real estate using leverage.The penalties for prohibited transactions are less severe.Tax deduction, this plan allows contributions up to $61,000 per year per participant.