Show active parent's listed children when viewing it children When I view a entry that's belong to a small group of children with a parent assigned. When I view the child, I want to display list all children that the child assigned to? 

Is there anyway to achieve this method?
I think you're looking for the getSiblings() method. This will return all sibling entries for the current entry. You can also use getParent() to get the parent entry and getChildren() to get the entries child entries.

An example implementation is below:

{% set siblings = entry.getSiblings() %}
{% set parent = entry.getParent() %}
{% set children = entry.getChildren() %}

{% if parent|length %}
  &lt;h2&gt;Parent&lt;/h2&gt;
  &lt;p&gt;{{ parent.title }}&lt;/p&gt;
{% endif %}

{% if siblings|length %}
  &lt;h2&gt;Siblings&lt;/h2&gt;
  {% for entry in siblings %}
    &lt;p&gt;{{ entry.title }}&lt;/p&gt;
  {% endfor %}
{% endif %}

{% if children|length %}
  &lt;h2&gt;Children&lt;/h2&gt;
  {% for entry in children %}
    &lt;p&gt;{{ entry.title }}&lt;/p&gt;
  {% endfor %}
{% endif %}


More info on these methods can be found in the EntryModel documentation.