Add add type argument to children aggregation.

src/Aggregation/ChildrenAggregation.php

@@ -35,6 +35,17 @@ class ChildrenAggregation extends AbstractAggregation
         return $this->children;
     }
 
+    /**
+     * @param string $name
+     * @param string $children
+     */
+    public function __construct($name, $children = null)
+    {
+        parent::__construct($name);
+
+        $this->setChildren($children);
+    }
+
     /**
      * Sets children.
      *