Center child divs inside parent div

Try using display: inline-block and text-align: center

.parent {
    width: 100%;
    border: 1px solid blue;
    text-align: center;
}

.child {
    display: inline-block;  
    border: 1px solid red;
    margin: 2px;
}

jsFiddle: http://jsfiddle.net/qdHH3/3/

.child {

    $box-size: 100%;
    
    width: $box-size;
    height: $box-size;

    display: flex;  
    justify-content: center;
    align-items: center;
}

Flex solution: set justify-content: center; to the parent element:

.parent {
  display: flex;
  justify-content: center;
  /* align-items: center; /* To align vertically, if needed */
}

.parent, .child { border: 1px solid #000; }

<div class="parent">
    <div class="child">child1</div>
    <div class="child">child2 - center us child divs! :)</div>
    <div class="child">child3</div>
</div>