Most traditional brokerage/IRA firms like Schwab, Fidelity, etc do not custody alternative assets due to the complexity and regulations involved in asset review, reporting, valuations, etc.

Sometimes, families are almost forced into buying houses with more bedrooms when there are custody battles involved.

I agree with @Curt Smith, checkbook control is almost essential depending on your type of investing and a Solo 401k sure beats an IRA if you are eligible.Contributions limits are roughly 10x higher, there is no custodial requirement for the 401k, you can take participant loans, you don't need the additional expense and administration of an LLC to have checkbook control, there is a built in-Roth component, a spouse can participate, there are additional tax benefits, there is generally greater privacy, and the plans are often quicker to setup and cost less money over time especially compared to most IRA LLCs.

Lue C.If you have self-employment activity, you may want to consider a Solo 401k instead of an IRA.Contributions limits are roughly 10x higher, there is no custodial requirement for the 401k, you can take participant loans, you don't need the additional expense and administration of an LLC to have checkbook control, there is a built in-Roth component, a spouse can participate, there are additional tax benefits (UDFI exemption), there is generally greater privacy, and the plans are often quicker to setup and cost less money over time especially compared to most IRA LLCs.

An alternative to custodial SD IRA is Checkbook IRA, which allows you to have a checkbook control over your retirement funds just like Solo 401k does. 

Compared to an IRA, contributions limits are roughly 10x higher, there is no custodial requirement for the 401k, you can take participant loans, you don't need the additional expense and administration of an LLC to have checkbook control, there is a built in-Roth component, a spouse can also participate, there are additional tax benefits, generally greater privacy, and the plans are often quicker to setup and cost less money over time especially compared to most IRA LLCs.

I realized I have to do something or else the tenant could have children "squatting" there that aren't even in their custody.

Compared to an IRA, Solo 401k contributions limits are roughly 10 times higher, there is no custodial requirement for the 401k, you can take participant loans, you don't need the additional expense and administration of an LLC to have checkbook control, there is a built in-Roth component, a spouse can participate in the same plan, there are additional tax benefits (UDFI exemption), and the plans are often quicker to setup and cost less money over time especially compared to most IRA LLCs.

@JennyMarie GreenoughFirst, you need a self-directed custodial account allowing you to buy alternative investments.

Interesting idea, my son at 18 mo did modeling, and continued for 10 more years, his money went into custodial account.