To me, since the DB is there to serve the app (which is there to serve the user), the lookup/enum decision mostly depends on whether the list is defined before

To me, since the DB is there to serve the app (which is there to serve the user), the lookup/enum decision mostly depends on whether the list is defined before build time (> enum) or after (> lookup). US states are probably a solid "before", so you get the added value of easily materializing a validator in the app code. Children IDs sound a bit more dynamic.