what is the {date of birth} for {Pedro I} has {child} as {Pedro II of Brazil} ?
What is the date of birth of Pedro I (whose son is Pedro II of Brazil)?
When was Pedro I(who has a son, Pedro II of Brazil) born?
SELECT ?v WHERE { ?s property:date_of_birth ?v . ?s property:child resource:Pedro_II_of_Brazil . resource:Pedro_I_of_Brazil property:child ?s }
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q939> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P40> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <http://wikidata.dbpedia.org/resource/Q156774> .
?statement <http://www.wikidata.org/entity/P569> ?value. 
} 

SELECT ?value WHERE { wd:Q939 p:P40 ?s . ?s ps:P40 wd:Q156774 . ?s pq:P569 ?value}