CB

Actually, it would be a feature also useful for normal users (e.g. family parents creating accounts for their children).
So it would be very useful to store the user-id of the creator of the account.

In that case, front-end fields of registration form would be more useful than backend ones.

Finally, blocking of creator of accounts should in that case probably also block the created accounts (at least if user is not a moderator?)

Default of course: Off.

That sounds more like some sort of account linking functionality and would probably just be a separate core field for that. Then that behavior would all just be handled automatically during the various actions (e.g. parent account closed then also close child accounts with unlimited chaining potential). This wouldn't necessarily be dependent on frontend registration, but would be available to work in all cases (e.g. set by CB Auto Actions or in backend user edit too).