Well, I found the solution myself.In this case, the non-responsive child domain did not exist anymore. It was defined under DSC@servername Directory Service connector as an Active Directory LDAP entry:
I re-created a new (and fake) Active Directory Integrated DNS zone with the mychilddomain.mydomain.com name and a HOST entry for that zone.
Then I went back to QMC and succesfully deleted the LDAP entry. Then I deleted the zone in DNS.