Get all subcategories' id as array

I know there's a ton of posts on subcategories subject here but I've checked most of them and none seem to answer my question.

I have "categories" table with records are either a root category or related to a root category by parent_id like this;

ID  |	NAME          |	PARENT_ID
------------------------------------
1	|	root          |	null
2	|	child 1       |	1
3	|	child 1.1     |	2
4	|	child 1.1.1   |	3

What I want to achieve;

subCategoriesArray(1); 		// [2, 3, 4]

Reason : I'm making a category edit page and I want users to be able to change a category's parent. In order to prevent a category from being a child of its own child, I need to eliminate its subcategories in the category selection box.

My code so far :

function subCategoriesArray($id){
    $data=[];
    $children = Category::whereParentId($id)->get();
    foreach ($children as $child){
        $data[] = $child->id;
        $data = array_merge($data, subCategoriesList($child->id));
    }
    return $data;
}

it only returns category's direct children for some reason.

Category Model :

class Category extends Model
{
    use HasFactory;
    use SoftDeletes;

    public function children(){
        return $this->hasMany(Category::class, 'parent_id')->with('children');
    }

    public function parent(){
        return $this->hasOne(Category::class, 'id', 'parent_id');
    }

}