parent:children is no accessible

I am trying to show the subnavigation on the subpages sidebar of my website. But even tough the dump of parent has children i cannot access them. Or how else are we supposed to create subnavigations on pages?

        <aside class="md:w-1/3 px-4 position-sticky">
            <h3 class="uppercase">{{ parent:title }}</h3>
            <nav>
                {{ nav:parent:children }}
                    <a class="block border-b border-gray-300 pt-4 pb-2 hover:text-primary-500 focus:text-primary-500 duration-200 ease-in-out"
                       href="{{ url }}">
                        {{ title }}
                    </a>
                {{ /nav:parent:chilren }}
            </nav>
        </aside>