Enable propagate host downtime commands for Icinga 2 >= 2.6.0 and Icinga 1.x

fixes #10774

modules/monitoring/application/forms/Command/Object/ScheduleHostDowntimeCommandForm.php

@@ -33,6 +33,9 @@ class ScheduleHostDowntimeCommandForm extends ScheduleServiceDowntimeCommandForm
             )
         );
 
+        if (! $this->getBackend()->isIcinga2()
+            || version_compare($this->getBackend()->getProgramVersion(), '2.6.0', '>=')
+        ) {
             $this->addElement(
                 'select',
                 'child_hosts',
@@ -49,6 +52,7 @@ class ScheduleHostDowntimeCommandForm extends ScheduleServiceDowntimeCommandForm
                     'value'         => 0
                 )
             );
+        }
 
         return $this;
     }