Hi Gernot,
1) tags on the child like is_in or addr:street/addr:city
I believe that info is currently used.
2) place and boundary polygons for the "parent"
I don't think that's currently used.
3) relations
Also, not used at the moment. I wrote the road-name-pois code and it simply associates roads with the nearest "city". It's very dumb but quite effective. I would have thought that having place/boundary polygons would be useful for determining the road names for cities. I haven't thought about general address info but relations sound useful because you could use a relation to associate a bunch of houses with a street, or a bunch of streets with a place. The street address search is semi-functional at the moment. Personally, I don't think it will improve much until we understand how to generate the extra data required. Unfortunately, that isn't going to happen soon due to lack of collective knowledge. Cheers, Mark