Cascade custom attribute to child belongsTo Model

I have a Parent model with multiple Child models. The child models perform a calculation based a factor inside the parent model.

The factor can be changed by the user. I set the factor overwrite inside the controller, but the child classes don't get the altered parent object but the orignal from the database.

Models:

class Parent extends Model
{
    public function childs()
    {
        return $this->hasMany('App\Child');
    }

    public function getFactorOverwriteAttribute($value) {
        if (is_null($value)) {
            return $this->factor;
        }

        return $value;
    }
}

class Child extends Model
{
    public function parent()
    {
        return $this->belongsTo('App\Parent');
    }

    public function getAmountAttribute($value)
    {
        // doesn't recognize `$parent->factor_overwrite = $user_value` in the controller.
        return $value / $this->parent->factor * $this->parent->factor_overwrite;
    }
}

Controller:

class ParentController extends BaseController
{
    function show($id)
    {
        $parent = Parent::find($id);

        if (request()->has('factor_overwrite')) {
            $parent->factor_overwrite = request()->factor_overwrite;
        }

        // doesn't use the overwrite
        dd($parent->childs->first()->amount);
    }
}

Is there a best practice to hand down such a value? I'm using Laravel 5.4.

via Chebli Mohamed