Thinking of becoming rep payee for my parents - need advice.

Freezing credit reports will not completely prevent lines of credit, it will prevent most.  Freezing credit reports does exactly that.  But there's no law or other requirement that a creditor look at a credit report before extending credit.  I've gotten credit cards in the past with no credit pull.  Sometimes a bank with which you have a long term existing relationship with will not bother.  Other examples are lenders that cater to people with little or no credit history, young people starting out, that also will not do a credit pull since there is nothing to pull.

Don't get me wrong, freezing credit is a good thing in general.  Mine has been frozen for more than a decade, but it's not a air tight solution.  Also, be aware that their are more than the 3 big credit bureaus most people think of.  The last time I checked their are 5 widely used ones.

How do you freeze their credit as a DPOA?

Rep payee does not prevent opening new credit accounts.freezing their credit at each of the credit bureaus is the only way and parents can revoke that if competent.