American_the_George So "get an iPhone" is my only option here?
At present the GrapheneOS user-profile system is pretty directly inherited from AOSP, which is (a) defined by Google and (b) somewhat in flux. So at present there are many features that some people would like that are not supported.
As just one example, it's not really feasible to set up a user profile for a child that is administered by a parent, because the OS can't completely boot without the administrative user unlocking it.
In answer to your question, it is possible to set up a limited profile. But if the device reboots, that limited profile can't be accessed until the owner profile has been activated. It is plausible that at some point in the future this restriction will go away, but at present it is in force.