How do you dynamically center a div inside another div element relative to content?

I am currently trying to specifically position multiple divs, inside of another div, which is itself centered in another div.

To get a visual perspective of what I'm trying to do:

I have a parent div with a specific height and width. Inside of that div element should be another div (white border), which is centered. Again, inside of that div should be a number of specifically positioned (green) divs. The green divs should be created and appended by a javascript function.

I now want the centered divs' height and width dynamically change, when I put my green divs inside.

My current code is:

HTML:

<div id="grandparent">
    <div id="parent">
        <!-- <div class="child"></div> -->
        <!-- <div class="child"></div> -->
        <!-- <div class="child"></div> -->
        <!-- <div class="child"></div> --> (commented because they're not yet in there)
    </div>
</div>

CSS:

#grandparent{
    height: 130px;
    width: 145px;
    background-color: #000000;
    border: 3px solid #00ffff;
    display: table;
}

#parent{
    display: table-cell;
    vertical-align: middle;
}

.child{
    height: 23px;
    width: 23px;
    float: left;
}

javascript:

for(var i = 0; i < number; i++){
    var top = ...
    var left = ...
    var child = $("<div class='child'></div>");
    child.css("top", top);
    child.css("left", left);
    $("#parent").append(child)
}

So far my content was based on this guide: http://www.vanseodesign.com/css/vertical-centering/

How can a div's size change according to its content? Will a div then stay centered, if you change this content?