JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
    <div id="box1"></div>
    <div id="box2"></div>
</div>

CSS

#outer, #box1, #box2 {
    position: absolute;
}
#outer {
    left: 50px;
    top: 50px;
    width: 200px;
    background: #aaa;
}
#box1, #box2 {
    width: 20px;
    height: 20px;
    background: #f00;
}
#box1 {
    left: 10px;
    top: 40px;
}
#box2 {
    left: 50px;
    top: 20px;
}

JavaScript

var element=$("#outer"); // element is a jQuery object
var max=0;
element.children().each(function(index, child) {
    child=$(child);
    var childPos=child.position();
    var childMax=childPos.top+child.outerHeight();
    if(childMax>max) {
        max=childMax;
    }
});
element.css({height: max+'px'});