You can install the “Exclude Pages from Navigation” plugin and use that on the child pages of the “About” page.
Having said that, so that others may see the child pages of the “About” page, you might like to hardcode those links in the “About” page either as unordered list or as a horizontal link
Unordered list – paste below in HTML view
<ul>
<li><a href="https://www.erikkok.nl/about/doel/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/tsjernoby/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/trainyard/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/moscow/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/fraser-island/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/turkey/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/ecvb-powerplant/" title=""></a></li>
<li><a href="https://www.erikkok.nl/about/chateau-vurstjen/" title=""></a></li>
</ul>
or as regular links
<a href="https://www.erikkok.nl/about/doel/" title=""></a> | <a href="https://www.erikkok.nl/about/tsjernoby/" title=""></a> | <a href="https://www.erikkok.nl/about/trainyard/" title=""></a> | <a href="https://www.erikkok.nl/about/moscow/" title=""></a> | <a href="https://www.erikkok.nl/about/fraser-island/" title=""></a> | <a href="https://www.erikkok.nl/about/turkey/" title=""></a> | <a href="https://www.erikkok.nl/about/ecvb-powerplant/" title=""></a> | <a href="https://www.erikkok.nl/about/chateau-vurstjen/" title=""></a>