Апатий Черный

$(document).ready(function () {
    $('#list > li').click(function (event) {
        $(this).children("ul").slideToggle();
        event.stopPropagation();
         $(this).children("ul").children("li").children("i").addClass('active');
    });
});

.active{
transition: 1s;
transform: rotate(90deg);
}