.nav-folder-children .nav-file-title-content:first-child::before { content: '🗒 '; }
.nav-folder-children .nav-folder-title-content::before { content: '📂 '; }

Using Wingdings or IcoMoon

.nav-folder-children .nav-file-title-content:first-child::before {
    content: "\e924  ";
    font-family: 'IcoMoon-Free';
  }
  .nav-folder-children .nav-folder-title-content::before {
    content: '\e930  ';
    font-family: 'IcoMoon-Free';
  }