Couple things:
1. This line needs to actually link to the file in your child theme:
<link href="dist/hamburgers.css" rel="stylesheet">
So instead of dist/hamburgers.css
, you would put yoururl.com/to/child/theme/hamburgers.css
.
2. Use this as your function:
add_action( 'generate_inside_mobile_menu', function() {
?>
<div class="hamburger hamburger--collapse">
<div class="hamburger-box">
<div class="hamburger-inner"></div>
</div>
</div>
<?php
} );
3. Add this CSS:
.menu-toggle .mobile-menu,
.menu-toggle:before {
display: none;
}