Javascript OOP callback "this" apply

i have a problem when creating callback on my own EACH function.

i'm using OOP way to accompolished it.

basically, i create my own javascript library that mimicry JQUERY habits.

check this fiddle out on action : https://jsfiddle.net/6pk068ep/1/

<div class="parent">
    <p class="child">child</p>
</div>
<div class="parent">
    <p class="child">child</p>
</div>

The JavaScript:

"use strict";

(function(window) {

    var i, $, $Obj, ele; // global variable

    $ = function(el, length) {
        return new $Obj(el, length); // create new object
    };

    $Obj = function(el, length) {

      ele = document.querySelectorAll(el); // get selector element 

        this.length = ele.length; // count the length of ele

        for (i = 0; i < this.length; i++) {
            this[i] = ele[i]; // loop it
        }

    };

    $Obj.prototype = { // object prototype

       each : function(fn) { // create method each just like jquery

       var arr = []; // create array to store value

       for (i = 0; i < this.length; i++) {          
         arr.push(this[i]); // push it to arr variable
         }

       fn.apply(this, arr); // IS THIS THE CORRECT WAY TO APPLY IT? 
       return this; // return this, so, it's chainable for other method

      }

    };

window.$ = $; // make dollar sign global object

})(window);

$(".child").each(function() {

    console.log(this);
    this.style.color = "red"; // not working, what's wrong with my code? is it because my "apply" wrong?

});

how to get those .child style color to red?

what's wrong with my code?

thanks in advance...