Got it working!
There’s no need to use this $image_thumb = $icon['sizes']['full'];
Just echo $icon and Return Value should be Image URL nor Image Object.
Thanks again Jonathan for help.
<?php
$current_term = get_queried_object();
$args = array(
'parent' => $current_term->term_id,
'orderby' => 'slug',
'hide_empty' => false
);
$child_terms = get_terms( $current_term->taxonomy, $args );
?>
<ul>
<?php foreach ($child_terms as $term) { ?>
<?php
$icon = get_field('icon', $term->taxonomy . '_' . $term->term_id);
?>
<li>
<h3><a href="<?php echo get_term_link( $term->name, $term->taxonomy ); ?>"><?php echo $term->name; ?></a></h3>
<img src="<?php echo $icon; ?>" class="icona" alt="" />
</li>
<?php } ?>
</ul>
