*{
    box-sizing: border-box;
}

body {
    background-color: skyblue;
}

#padre {
    width: 100%;
    height: 250px;
    background-color: blueviolet;
    display: flex;
    flex-wrap: nowrap;
    padding: 10px;
}

#hijo1 {
    width: 20%;
    height: 230px;
    background-color: palegoldenrod;
}

#hijo2 {
    width: 100%;
    height: 230px;
    background-color: palevioletred;
    margin-left: 10px;
    padding: 10px;
}

#nieto1 {
    width: 100%;
    height: 210px;
    background-color: palegreen;
}