display: flex;
 align-items: center;

#boss {
    border: 1px solid #000;
    width: 300px;
    height: 300px;
    display: flex;           /* moved here from child div */
    align-items: center;     /* moved here from child div */
}

#kids {
    margin: 0 auto;
    border: 1px solid #000;
    width: 150px;
    height: 150px;
}

#boss {
    border: 1px solid #000;
    width: 300px;
    height: 300px;
    display: flex;
    /* align-items: center <-- REMOVE */
}

#kids {
    margin: auto;            /* adjusted */
    border: 1px solid #000;
    width: 150px;
    height: 150px;
}