SELECT    ?child  (GROUP_CONCAT (?string) AS ?strings)
WHERE {
    SELECT *
    {
     ?child  wdt:P279 wd:Q7275 .
     ?grandchild wdt:P279 ?child .
     BIND (STRAFTER(STR(?grandchild), "http://www.wikidata.org/entity/") AS ?string)
    } ORDER BY ASC(?string)
}   GROUP BY ?child
LIMIT 20