Edit in JSFiddle

<div class="container">
    <div class="child1">Child 1</div>
    <div class="child2">Child2</div>
</div>

.container{
    background-color:#ccc;
    height: 500px;
    display: table;
    width: 100%;
}

.container .child1 {    
    display: table-row;   
    background-color: green;
}

.container .child2 {
    display: table-row;    
    height: 100%;
    background-color: red;
}