Sub-Menus without dropdowns

Is there any way to not use the drop down menus, and rather have the primary menu show the subpages/children of the current page under it in the secondary menu? And not by hard-coding this in the header.php file, but something that can be enable/set in the menu or theme management itself.

I am working on a website that needs the subpages to always be visible for accessibility issues.

(i saw an earlier topic discussed, but this problem was not resolved)