How can I optimize or reduce the time complexity of the following code in Laravel 5.2.45?

In users table I have following data.

id   name    parent_id 
1     A           0
2     B           1
3     C           2
4     D           3
5     E           4
6     F           1

A have four level children. E is also children of A. E is the last, it has not any children. Now I can fetch all the children of any parent like the following code.

function getChildren($Id){
   $cache = [];
   $chList = [];
   $allUser = User::orderBy('parent_id', 'asc')
        ->orderBy('parent_id', 'asc')
        ->get();
   foreach($allUser as $user) {
    $cache [$user->parent_id][] = $user;        
   }
  $children0 = empty($cache [$Id]) ? [] : $cache [$Id];
  #first level child
  foreach($children0 as $child1) {

    $chList[$child1->id] = $child1;

    if(!empty($cache [$child1->id])) {

        #Second level child
        foreach($cache [$child1->id] as $child2) {

            $chList[$child2->id] = $child2;

            if(!empty($cache [$child2->id])) {

                #third level child
                foreach($cache [$child2->id] as $child3) {

                    $chList[$child3->id] = $child3;

                    if(!empty($cache [$child3->id])) {

                        #fourth level child
                        foreach($cache [$child3->id] as $child4) {
                            #can not be parent
                            $chList[$child4->id] = $child4;
                            
                        }
                    }
                }
            }
        }
    }
}

return $chList;
}

I can get the actual result by this code. But for huge data it is time consuming. Can any one help me to reduce time complexity or optimize the code?

via Chebli Mohamed