This code is 1 of 2 parts needed to nest sub menu items in Divi. Place this code in style.css file in your child theme directory.

Nesting Sub Menu Items in Divi's Mobile Menu (CSS)

#main-header .et_mobile_menu .menu-item-has-children>a {
    background-color: transparent;
    position: relative;
}

#main-header .et_mobile_menu .menu-item-has-children>a:after {
    font-family: 'ETmodules';
    text-align: center;
    speak: none;
    font-weight: normal;
    font-variant: normal;
    text-transform: none;
    -webkit-font-smoothing: antialiased;
    position: absolute;
}

#main-header .et_mobile_menu .menu-item-has-children>a:after {
    font-size: 16px;
    content: '\4c';
    top: 13px;
    right: 10px;
}

#main-header .et_mobile_menu .menu-item-has-children.visible>a:after {
    content: '\4d';
}

#main-header .et_mobile_menu ul.sub-menu {
    display: none !important;
    visibility: hidden !important;
    transition: all 1.5s ease-in-out;
}

#main-header .et_mobile_menu .visible>ul.sub-menu {
    display: block !important;
    visibility: visible !important;
}