How to center horizontally div inside parent div

I am assuming the parent div has no width or a wide width, and the child div has a smaller width. The following will set the margin for the top and bottom to zero, and the sides to automatically fit. This centers the div.

div#child {
    margin: 0 auto;
}

.parent-container {
  display: flex;
  justify-content: center;
  align-items: center;
}

.child-canvas {
  flex-shrink: 0;
}

<div id='parent' style='width: 100%;text-align:center;'>
 <div id='child' style='width:50px; height:100px;margin:0px auto;'>Text</div>
</div>

Just out of interest, if you want to center two or more divs (so they're side by side in the center), then here's how to do it:

<div style="text-align:center;">
    <div style="border:1px solid #000; display:inline-block;">Div 1</div>
    <div style="border:1px solid red; display:inline-block;">Div 2</div>
</div>