I think it's possible. You need to catch the parent select's change and update the child select accordingly like this:

class MyBoard extends \koolreport\dashboard\Dashboard
{
    protected function widgets()
    {       

        return [
            ...
            Row::create([
                [
                    Html::label("Parent")->style("font-weight:bold"),
                    BSelectParent::create(),
                ],
                [
                    Html::label("Child")->style("font-weight:bold"),
                    BSelectChild::create(),
                ],
    }
}


class BSelectParent extends \koolreport\dashboard\inputs\BSelect
{
    ...

    protected function actionChange($request, $response)
    {
        $this->sibling("BSelectChild")->update(); // update the child select
    }
}


class BSelectChild extends \koolreport\dashboard\inputs\BSelect
{

    protected function onInit()
    {        
        $parent = $this->sibling('BSelectParent')->value() ?? null; // scalar or array value depending on whether BSelectParent is single or multipel select
        $selectData = ...;
        
        $this
            ->props([
                "dataSource" => $selectData,
                "dataBind"=>[
                    "value" => ...,
                    "text" => ...,
                ],
            ]); 
    }
}