Php foreach loop wrapped every 2items with a row

$i = 0;
foreach ($children as $child) {
    $i++;

    // your code goes here 
    if($i % 2 == 0) { 
        echo "</div><div class='row'>";
        // reset the counter to 0 
        $i = 0 ;
    }
}

use proper logic

    if ($i % 2 == 0) { 
        echo "</div><div class='row'>";
    }
    $i++;

First check if its mod by 2 or not (Gives 0 value after MOD), then close div , open new.

Now increase counter . Because for the first time , i will be 0 , then you increment it and then you use logic. So in short counter shoul be incremented at the end only not in between before you do any operation/logic.

Updated

Use code as it is **: issue was you have i++ and your condition in 3rd else if which never executed. So took it outside All and just before foreach.

<div class="puffar">
    <?php
    //Set up the objects needed
    $my_wp_query = new WP_Query();
    $all_wp_pages = $my_wp_query->query(array('post_type' => 'page'));

    //Get children
    $children = ($post->post_parent) ? get_page_children($post->post_parent, $all_wp_pages) : get_page_children($post->ID, $all_wp_pages);

    $i = 0;
    //Build custom items
    echo "<div class='row'>";
    foreach ($children as $child) {
        ?>

        <div class="col-sm-6">
            <div class="puff">
                <div class="puff-image-holder">
                    <?php echo get_the_post_thumbnail($child->ID, 'full'); ?>
                </div>
                <fieldset class="linedHeadline hlmedium">
                    <legend><?php echo get_the_title($child->ID); ?></legend>
                </fieldset>
                <?php echo get_field("puff_introtext", $child->ID); ?>
                <?php
                $values = get_field('puff_lanktext', $child->ID);
                if (get_field("popup_eller_lank", $child->ID) == "popup") {
                    ?>
                    <fieldset class="linedHeadline hlmedium">
                        <legend><a class="linktopage open-popup"
                                   href="<?php echo get_page_link($child->ID); ?>"><?php echo get_field("puff_lanktext", $child->ID); ?> </a>
                        </legend>
                    </fieldset>
                <?php
                } elseif (get_field("popup_eller_lank", $child->ID) == "extern") {
                ?>
                <fieldset class="linedHeadline hlmedium">
                    <legend><a class="linktopage"
                               href="<?php echo get_field("puff_lank", $child->ID); ?>"><?php echo get_field("puff_lanktext", $child->ID); ?> </a>
                    </legend>
                    <?php

                } else {
                }
                ?>
            </div>
        </div>
    <?php
        if ($i % 2 == 0) {
            echo "</div><div class='row'>";
        }

        $i++;
    } ?>
</div>
</div>

First set $i=0;

if ($i % 2 == 0) { 
        echo "</div><div class='row'>";
    }
    $i++;