if you want to remove the word "Children categories", then find this line in category.php
<h3><?php echo JText::_('K2_CHILDREN_CATEGORIES'); ?></h3>
and comment it out like this:
<!--<h3><?php echo JText::_('K2_CHILDREN_CATEGORIES'); ?></h3>-->
if you are not using template override, you will find the category.php file in components/com_k2/templates/default
Otherwise, look for this file in templates/yourtemplate/html/com_k2